Not a government site, but some discussion of the process:
https://politics.stackexchange.com/questions/12219/can-a-bill-that-has-passed-the-us-senate-or-house-expire
"Bills expire at the end of the Congress in which they were proposed if not sent to the President to be signed. Bills that are sent to the President either expire (if Congress is out of session) or are implicitly passed (if Congress is in session) within ten days (not including Sundays)."