I'm going to get laser for a small tear in my retina. How long after that can I start doing intense and heavy workouts?

Wait a month. The purpose of laser for a retinal tear is to produce a scar to stick the retina to the underlying tissue. This takes 2-4 weeks to occur.
Immediately. No reasons why not although many docs will advise against, however, your eyes develop significant forces internally when sleeping. Every night we all go through rem (rapid eye movement) sleep. The eyes move back and forth rapidly generating tremendous forces within the vitreous. This is my rationale with my own patients, your doc may disagree.