Depends on how much Suboxone you were taking and for how long, and also how much heroin you'll be doing. If you were taking 8mg or more Suboxone, you probably need to wait a full 48 hours or more to truly "feel" your average-sized dose of heroin, but people vary on this quite a bit. At lower doses of Suboxone, like 2mg and under, the waiting time can be a lot less.