I work out in the late afternoon, and usually will have a snack if it's a work day. How long I wait depends on what type of workout I'm doing, and how much and what I ate. If I'm doing an ab intensive workout, or a workout that includes a lot of jumping, I stick with a light snack (maybe even a soy protein meal replacement), or if I eat more, I'll wait longer--about an hour.
If I'm doing a weight workout, I can usually eat a bit more and/or wait less time. Sometimes as little as 15 minutes, but usually a half hour.