i have one and thought it was about 5 hours ish! I left one in for 24 hours once and it did go a bit of a funny shape but went back to normal again, I wouldn't leave them in too long just in case. These things are fab! I have four but could do with a couple more as I have 4 guineas (in two cages) and a bunny so I really need one per cage (3) in freezer and one per cage in the cages at same time! I saw them for only 6.99 in P@H the other day, thought it was very reasonable!