Consider your setting. Where do you want the listener to picture this vocalist when they play back your recording? Much like setting a stage for a movie, reverb can set the stage for the singer. Yet, just as the set should never upstage the actor in a film, the effects on a recording should never overshadow the vocals themselves.
Learn "arrival time gap" basics. Reverb is used to place a sort of echo on the vocals being recorded. The amount of time between the moment that the original sound reaches the hearer and the moment that the reverberation reaches that person, is considered the "arrival time gap." This gap is longest in large open areas, but may be completely unnoticeable in small enclosed spaces.
Study the science of sound. Knowing how sound travels, how and when it vibrates both inside and outside of the ear, and how it responds to different circumstances can help you become an expert at using reverb.
Balance your use of reverb. Too much reverb makes a vocalist sound phony, and is often used to hide a mediocre natural sound. On the other hand, too little reverb can make a recording sound hollow and empty.
Experiment until you have a sound that you feel is right. Many sound technicians rely mainly on their ears for music, and their personal instinct to decide exactly how much reverb is needed.