Yeast cells are some tough lil critters. If it's too hot,they die. If it's too cold,they go dormant. I've read stories on here where beer sat in primary for 5 months or more,& still came out good.
By & large,this question is def in the gray area. There lifespan is relavent to the conditions they're in. For most ales' time in primary,I don't woory about it.