depends on age and how healthy they are to begin with. LG if they are healthy adults can survive a while without heat but they will definitely not be healthy after wards...
I'm no expert by any means, but we had a leopard gecko for several months with no heat (not more than the low 70's), and she's the easiest leo I have to this day.
My adult male was without heat for about a year & a half. My room stayed around 85-87 during the day when noone was home & the ac was off. When the ac was on it might have been around 7o....in the winter i moved his tank to where he'd get heat from the vent. I would also wrap a towel around his tank when we didn't have the heater on. He stayed fat & had no shedding problems. although I would never do that with the new geckos we have now. They seem 90% more fragile than he was. I'm just glad I had a very tolerant gecko as a first pet.