GC.com prompts the user to notify the TB owner after 14 days of inactivity. On the my account page it gives the following message:
"* No activity in the last 14 days - Please drop off this travel bug in a new cache or contact the owner as a courtesy. If the Travel Bug has been lost, the owner can change the bug's status to an unknown location."
I know this because I have had a bug for 4 weeks now (due to unforseen circumstances). I emailed the owner explaining and will drop off asap.
If it was my bug and I hadn't heard after two weeks I would drop a short note to check on well being.