It all depends on how many ships you can afford to upkeep, which for me so far isn't that many. Heh. I would venture to say that you shouldn't expect to be able to support fleets on a SEIV or MOO2 scale until you've conquered a good majority of systems in the late game stage. But the beauty of STUN is that there's really no need to build massive fleets.
It was a little strange for me making the transition from SEIV to STUN, because I'm so used to massive fleet engagements myself. And really, I love the big fleets in those games. But the satisfaction from STUN comes from building just a few ships and watching them get better as they gain experience, special skills and abilities, and you research new toys to put on them. If you play well, and you fight smart, it's entirely possible to play through a STUN game without losing a single starship. And in fact, it tends to hurt quite a bit when you do lose one. It's not like SEIV where you can just queue up a replacement or two and no worries. Your interaction with your fleet is on a much more personal level in STUN. It's an altogether different experience and not a bit less rewarding for not having huge fleets than any other 4X game out there, IMO. This is anything but conventional 4X, and a very refreshing change of pace for me.