This is probably the #1 question that I get asked by many of the students. This is one question that has many different answers, but one underlying theme. You have to listen to the music and find the right balance for the reverb within the mix. There are a lot of factors that go into the decision of choosing the texture and depth of the reverb so that it is audible and felt, but not distracting. If the reverb is meant to be huge, use your judgement to make sure the space is the right fit for the mood of the music. Let me go over a few examples of what reverbs should do to add to the mix and not destroy them.
The first thing you need to ask yourself is does the track need reverb. Not all songs need to have reverb! This is something that gets overlooked by amateur engineers and producers. Just because it is there does not mean you need to use it. The simple test, if the mix sounds really good without reverb, then it does not need any reverb.
If you decide that the track could use some texture, depth, width, or space, then try a few different reverbs as a starting point. I usually set up about 4 - 5 different reverbs to give options, but it is not uncommon for me to use a blend of all the different reverbs. Sometimes all you need is one reverb to achieve the sound that is needed for the mix. When one is not enough, then it is time to start experimenting with the audible pleasures of multiple reverbs. Start with two and bring the levels down so the effect of the reverb is not audible. As the mix plays back, slowly bring in the first reverb until it is just barely noticeable. Then bring in the second reverb until they two compliment each other. You may need to adjust the size or predelay to get that silky smooth sound, but make small adjustments as you go along. Just go with your gut and let your ears tell you when the level of reverb is right.
The main thing to keep in mind is that the reverb is not the main focus of the song, so it should be in the background and enhance the song. Keep a modest amount of reverb in your mixes and your songs will start to get more attention. Music is organic and has life of its own. Be sure to let the music breathe and compliment it with the space of your reverbs. When in doubt, less reverb is what will work.