That is a very good question Emmanuel. I bet those chicks are growing like weeds! Generally it is around 4 weeks (or when they are fully feathered with no fluff left) unless
1) conditions are very cold or wet
2) they are still right under the light a lot
3) if you havent been raising the bulb a little each week (making it gradually cooler) and it is still quite low and they still go under it. If so, raise it the next 2 weeks and they should be good.
You know it is OK to switch it off when
1) the chicks never spend any time under it
2) even at night the chicks are right away from it
If you are nervous, switch it off when you are home and check them after an hour or two, or sometimes if it is around those temperatures, switch it off in the day and on at night for a couple of nights. Some chickens are more hardy than other and may apply differently.
Usually your chickens behaviour will tell you what to do. If they still often huddle under the light the coop is cold or drafty and they need the lamp longer. I hope that helps and it not too confusing!! ;-) Good luck!