Reverb is one of those tricky effects that are easy to use too much of. When I started mixing about a year ago I tended to use way too much, because if I backed it down I couldn't hear it. Now my ears have improved and I can detect a more subtle reverbs and how they alter a sound, but a good rule of thumb is to either bring the reverb up until you can just hear it (which is usually a little too much if you're trying to be subtle) and then back it off a tiny bit, until you feel like you can't quite hear it.
Then, after you've played through the track a few times, bypass the reverb and you should be able to hear the vocal (or whatever you used the effect on) pop out, it will sound very dry. A/B between the two and see if you need to slightly push the reverb up or down.
The point being that at first when you put a reverb on, you'll want to hear it to know you've done something, but it's easy to lay too much verb on a track. If you're going for a very wet sound, that's ok. If you're trying to be subtle, it's a good idea to back it off until you feel like there's too little verb on the track, and then A/B it and adjust from there after listening through the track a few times.
After a while things you put reverb on often in different songs (vocals, lead guitars, snare drum etc.) will become natural to you, and you'll have a much better feel for how you should proceed.
