Fresh mushrooms...?
 

Does anybody know how long they are safe to eat?

I just go by appearance; if regular button mushrooms are still fairly white and chalky looking, not brown and/or the least bit slimy, then they're OK. It helps to use a mushroom brush to clean them too, but only right before you use them. And they seem to stay fresh longer if you take them out of the plastic bag and put them into a paper bag, with a paper towel inside to absorb moisture. Or punch a few holes in the plastic box and throw a paper towel in there with 'em. Hope that helps...
Jan