I just put the "end of the month" for the date. I'm not going to hold people to that date. (But at some point -- e.g. 6 months later if it's still not done, then that's "not cool.")
Just to be be clear - it is *of course* up to the Project Manager how long to wait to accept or reject a task (the system only mandates that the date and time you offer must be >= to the suggested Delivery date and time).