When to upgrade fleet supply is probably the most strategic decision you make during the game. Do it too early, and you're economy will be weaker than it could have been. To late, and you may get steam rolled by someone who upgraded it sooner.
As Hyrim says, there is no consistent time of when to upgrade. As he says, sitting on a pile of cash means you're not putting all of your resources to use, so if you start accumulating resources faster than you can spend them, thats a good time. And even if you don't want to attack anyone (and you should, a fleet that's just sitting their isn't a very good investment), you'll need enough ships to counter whatever your foes can throw at them, so try to always keep as many ships as your opponent has. But beyond that it really depends on the game, your opponent, and how good you are at judging the situation and determining what it calls for.
