How many brew sessions on a propane tank?

I brew BIAB and usually get about (4) 5 gallon batches per tank. I've often thought about it but never really knew: What happens to my wort in the half hour that it would take me to get down to the store and back with a fresh tank of propane? Is it ruined because your boil has stopped in that 60 minute envelope of time that we call "the boil" or can you just fire it back up upon return and get it going again and proceed like normal?

 

I think the only casualty, if it could be called that, might be the hop schedule. Not sure how you would adjust for that.

 

I'm always afraid of running out in the middle of a brew day so I always top off or make sure that I have a back up just in case. I like to get all my ducks in a row a day before hand so I know once I start brewing, I won't have to go anywhere for supplies or anything like that. This past brew session, it seems like I went through a half of a 20lb tank for the strike, sparge and boil in 30 degree temps. I'm transitioning to an all electric HERMs set up so that I won't have to worry about propane anymore.