well there isnt technically much to do when you finish all the story missions unless you like grinding missions to get gear for the sake of it but its still enjoyable to just potter about in doing bits and pieces
I'm approaching the 2000 hour mark I believe, and I'll say that to be honest there are probably story missions I haven't done, largely because I found most of them poorly constructed and annoying. I've got quite a few top level characters to say the least and honestly I've taken to using the "shortcut" of running mirror universe incursions to max out new characters as fast as I can.
Despite how it sounds I'm fairly casual, I mostly keep playing to mess around with different ship configurations, see if I can accomplish missions in various ways, and oddly enough watch my fleet gradually increase in power and it's holdings expand (while far from the first to get there, we're just now approaching tour Tier 3 Spire and have everything else maxxed out, at which point we'll doubtlessly begin stockpiling for the next fleet holding).
I guess it's not for everyone though, truthfully I juggle multiple MMOs nowadays as opposed to playing one hardcore like I did with WoW. STO tends to keep me very occupied though as I've had a consistent presence in it longer than most games, and a quick visit to play with a few things rapidly turns into an exercise in "just one more mission or special task force".
To be fair, I do not think STO could sustain me (or many people) as their one and only MMO habit. A lot of players I know doing it also play Cryptic's other games at least part time (Champions, Neverwinter, etc...) and periodically bounce into other MMOs a lot like I do.
While I'm rambling though, I will also say that while I like the IDEA of the foundry it's too limited to have a lot of value. It's interesting to see what people can do with it, but at the end of the day it's by definition pointless as creators cannot assign loot, special awards, or other things for obvious reasons. The only real benefit to your character comes from general loot drops (of the sort shared by all mobs) and a dilithium/questionite/astral diamonds award for doing so many foundry missions as part of a daily quest. Since you can basically waste hours on a well made, and challenging adventure, and get absolutely jack for it (which is annoying if you were forced to use up tons of stims/healing potions/etc...). This sadly leads to the foundry turning into a hunt by people looking for quick missions that just barely quality for the daily quest. I tried my hand at design with "Neverwinter" and while it was tricky to figure out at first I managed to put together a fairly simple dungeon, and while feedback was mixed, at the end of the day I pretty much got told if I expected people to want to play it I needed to make sure it was a grindable cakewalk, not that many players ever really appeared for it. I may or may not try it again, I just found the effort of creating kind of thankless, and as a player know exactly where they were coming from.
I give Cryptic Kudos for the *idea* of the foundry, but it needs serious work as a concept. Cryptic needs to find a way to make The Foundry lead to more direct and meaningful awards for both players and creators, without unduly unbalancing the game, that is a tall order for sure, but even after tinkering with it on and off for years (mostly as a player) it strikes me as more of a novelty than anything anyone should be excited about.