Only on huge maps.  Once you get some experience, it's 3-hours tops, 1-hour typical for a normal-sized map.

The AI can be very unpredictable at times, often doing very well or very poorly seemingly at random.  Unless you're playing unfair difficulty or higher, there is a very real possibility that the AI is going to suck simply because it made a few very poor choices (for instance, trying to rush on a massive map, screwing over its long-term economy).

Do you have the expansion pack?  If so, I recommend turning on quick-start.  This gives you several structures and upgrades, including civilian infrastructure and extractors, for free at the start of the game.  This gets things rolling much more smoothly.

I'd also recommend trying the Progenitor Mothership (especially if you're playing on non-fast game speeds).  This will save you money and time that a missionary will otherwise cost you.  You can also split up your forces and colonize in two directions at once, which is more useful on faster game speeds where you have the cash to do this efficiently.

I've never found a feature so universally reviled as upkeep, and yet I've also never found a feature that's had such universally positive effects on gameplay.  It's very annoying, to be certain, but this minor nuisance creates a great deal of strategic depth that adds to the game as a whole. 

Anyways, it's very easy to play defensively and out-economy the AI, and this will easily take you to the highest difficulties.  However, you won't get too much mileage out of these tactics online, since human players strike early and much harder, so you will be unlikely to survive if you don't have a sufficient defensive force. 

This depends on the map; sometimes one large fleet is the way to go.  Other times, as you've realized already, you're better off splitting up.  This applies both to expansion, attacking, and defending.  Turrets work well against the AI, but human players simply go around them or attack them with long range units or strike craft.  This means they're not very useful in multiplayer and practically worthless if you don't have a fleet nearby to support them.

Your best defense is the scout; if you know the enemy is coming, you can redeploy your fleet accordingly.

Well, you don't need Starcraft-style micromanagement to succeed in Sins battles, but you do need some.  There are many abilities that can be left on autocast, but there are many others that simply cannot.  As Advent, repulsion (the upgraded ability of the Iconus Guardian) is your ace in the hole, and you will need to manually use it to be effective. 

At very least, toggle the autocast of capital ship abilities between battles so you control antimatter usage.  A very common problem I see with biggers is they leave autocast on and their capital ship blows all its antimatter on little skirmishes and then all their special abilities are completely depleted when it really counts.

What constitutes "high" income is very situational.  If you've got a big fleet and are constantly fighting an opponent, 50 credits per second is a very high income and very good.  If you have zero upkeep and you've been playing expansionist (and the enemy hasn't managed to stop you) this is actually a very low income.  In an average-case scenario, you'd start building your fleet before this point, and your income and fleet levels would grow together.

Oh yeah, you can hit very high values if you manage stay out of upkeep.  This is going to be limited by the size of the map and how big an empire you can make.  However, 100 credits per second is definitely doable on a normal-sized map.

The key to truly monsterous income is trade.  Trade ports get more profitable as you grow your trade route, and are unaffected by loyalty penalties.  This means that very large empires can produce absolutely massive amounts of money from trade.  In a best-case scenario on very large maps, I've heard of numbers as high as 600 credits per second.  I've personally hit over 300 in multiplayer.

You want to build the trade ports in such a way that you get the longest possible trade route--it can't circle over on itself or it could short-circuit the trade route.  Older threads from a two years ago provided a visual demonstration of how to construct a trade route; but you'd have to search for them.  Note that that the rules for what makes a trade route are different between vanilla Sins and Entrenchment/Diplomacy because in the expansions you have the possibility of building starbases and putting the trade ability on them, which will also count as trade ports for trade route purposes.

Also, you should fill all excess logistics slots on planets with trade ports--not just one per planet.  If a trade port at a planet generates X-income, building a second one at that same planet will generate 2X-income,  three will give you 3X-income at that planet, etc.  A desert can support as much as 9 trade ports (and even 10 if you are TEC and research the logistics slot upgrade--very useful).  I generally build my labs in safer areas away from the front lines and put the trade ports near the front, leaving one logistic spot free at each planet so that I can build a trade line.

If you ever get bored with the AI, feel free to log onto Ironclad Online to try it against human players; there are still new people out there.  You can also play comp stomps (people v. AI) if you're squeamish about playing against real people.  Oh, and it doesn't take forever to play it online.  Your typical 5v5 on Diplomacy only runs for about 1.5 hours today.  (How is that?  Because we play with Quick Start on and "Faster" speed settings.  Also the players on the losing team quit the game once it becomes obvious that they have lost, so you don't have to do drawn out, boring mop-up jobs.)

Ooohh, so, you can build more than one trade port in a planet?  I thought doing that would be redundant....

(unless people have 100 planets )

^Nope, building two trade ports will double the trade income for that planet, and so on until you run out of logistics slots. Refineries, on the other hand, do become redundant after enough of them are built.

Really appreciate all of those explanations, I can imagine it took you a while to write lol