If you mean being able to breath and having a heartbeat, then the answer is for years.
If you mean thriving, having a good life and not suffering, then a couple weeks tops.
well I'm gonna go ahead and assume your friend had a Heating/Basking bulb that gave off UVA. UVB is very critical to a beardy however its not deadly to them. I have a friend who also did this, and the downside to it was his beardy turned out to be very Dark in color where as bright and vivid. without uvb they'll eat not as much and look kind of "sad" often.
I just set Swordtail's timer for his bath and paused it so I could actually fill his soaking bowl up and he crawled over my phone and canceled the timer
Mirage came out of brumation on April 26. He was doing great. On May 2 he started acting funny. We just redid his tank, and he keeps going into one of his hides. He just lays there. He shows no intrest in food. HELP!