If you must ask how long a cannabis high lasts, you probably have not used cannabis very much. It is a sign you have not read up a lot, looked at your own health profile, and/or listened to budtenders and friends who know.
For starters, different cannabis strains affect different people uniquely. Their high is a function of their gender, height, weight, and bio-chemical predisposition. But you can imagine a generally typical person and how they might respond. So, with that caution in mind, you can and should explore how long a cannabis high will last.