Try checking Viruscan Console on that PC to make sure there are no other scan tasks and that the configuration of the managed scan matches what you set up in ePO. If things look different, check the agent logs on that PC to make sure it is communicating with ePO properly and getting and enforcing policies. If all that checks out, I don't have any other suggestions, but maybe someone else will. I've never set up a scan with a time limit like that.
The only other thing I can think of is to make sure that these fields aren't set to a lenghty amount of time.
Also, check the to make sure the local machine has the proper time configured. If for some reason it's 4 hours behind, it wouldn't think it's supposed to stop the task.
10x I will give it a try and let ya know.
When you check the log are you verifying that the ODS task you see is the one that was kicked off by the task, and not by a user who started his own scan? If you're using FUS it may also be that another user kicked it off before the user who noticed it logged in and found the task running?