5 minutes
first thing you should notice is your chest feeling like it is caving in. It sounds like an unpleasant experience but its really not and once you associate it with the high that follows you start to enjoy it.
a lot of times I would snort some bags, turn on sports radio until it kicked in, and then just wind up spacing out to that for like a half hour until I realized I was fully fucked up on heroin. It's very subtle when you snort, unlike when injected which gives you that initial rush but not a whole lot once that subsides