Unix Scripting?

I am creating a script that looks for a trigger file. If the trigger file does not arrive in 12 hours, I want the script to exit. What command do I use to do this? I think I don't want it to sleep for 12 hours because it has to look for the trigger. Maybe some Do While, but I don't know how to actually track the time. CRON kicks it off, but it stays alive if the trigger does not arrive. I want it to expire in 12 hours and exit. I obviously do not have a scripting depth, and am just winging it! Thanks for your help.