Fully up to you there. All of it is for plot, move the time as you see fit. Just set some initial like 4 hours, and when they are doing stuff every now and then push it back, or allow it to affect the characters, whatever better suits the plot.
If they aren't doing anything useful speed it up, maybe have a compartment blow up and kill some crew, that oughta get them moving on the ideas.
Radiation side effects are also something to consider, does it make you go crazy? or maybe your skin melts, or your hair falls out...all could be based on how much radiation is seeping in...as the GM it is all up to you.
"My feet hurt; I'm tired; We are stuck in this god damned damp cave...and WHAT THE HELL IS THAT SMELL"!?
-Gaming Session Player who shall remain unnamed