How much change to expect

I just bottled my Irish stout after fermenting for 17 days. I had a little left over and tasted it and it was pretty bitter, a little off, not sure how to explain it. I know It's flat and it still has atleast three weeks to age but I was just wondering can you even judge what your final beer will be by tasting it when bottling? I'm not worried or anything it would just suck to end up with a similar beer just carbonated three weeks from now.

There are still a LOT of changes to come for that beer. Just carbing changes it a lot, but honestly everything about it will change in the coming weeks.

I have read stouts take a little longer to condition.

Patience is your friend. Dark beers will continue to develop over time, even past the 3 week bottle conditioning stage. Make sure you keep some bottles to try in 6 weeks and at least a bottle or two to try at 6 months (or more!). It is hard to hold back, but if you don't, you'll discover that the beer was just starting to taste perfect while drinking the last bottle of the batch. It sucks when that happens.

Lol yeah I have a feeling that will happen, but I will try and save a few. Maybe most of the batch because the next brew shouldnt take long...hopefully my real problem becomes not having enough space for all the beers I brew so I'm forced to let certain ones age.