Master of Orion 2
Sword of the Stars
And many more games like that, all of them took many hours, if you choose to, have alot of planets in the game.
Removing the long term part in a space RTS would ruin, the part of the enjoyment of getting a system going. With hugh fleets in battle. Joining and breaking alliances, to your own benefits.
To my experiance

Though, i do see a problem, for those, who cant use 4 hours infront of a PC.
That is also why i have suggested a shorter version, if ppl just need to blow something up, in short time.
Like in the Total War games, you had an amount of point you could use for an army, and then you meet on the battlefield.
Something like that would be one way, to play a 10-20min game