How long after someone shoots up, will you know if they've overdosed?

Like, if my friend shot up an hour ago, is he out of the woods? Or is it possible to OD hours after?

It isn't possible to answer your question with the information provided. Here is a number that you can contact confidentially capable of answering your question:

Poison Control Centers 1-800-222-1222
Poison experts are at the poison center 24 hours a day, 7 days a week, 365 days a year. Their help is free and confidential.