How stable is an open vial of the 200/ml of depo-testosterone?

It should say on the bottle if it is a single use or multidose vial (I believe the 10ml vials are multi dose). If it doesnt say, one should assume it is single use. Most places I have worked, we discarded multi dose vial once they had been open 30 days but check with your pharmacy or supplier.

The problem isn't stability, its contamination. As long as you can be sure it wasn't contaminated and could grow bacteria it would be ok. It can't be refrigerated or I'd tell you to do that. Check with your pharmacist to be sure.