I was informed by the Silverton PO clerk that they had a 10-day time limit. When I picked up my general delivery after about 25 days, I was told she did me a favor by not shipping it back. She said she was trying to let everybody know about this "new" policy. I'm not sure how strictly they enforce this policy, so this is just a FYI.
To be safe, I believe Molas Lake Campground is an alternative or just pick up your package within 10 days in Silverton.
I had no problems in Breckenridge (7-day hold) or Salida (14-day hold). The USPS website states "each general delivery mailpiece is held for no more than 30 days," so this might be a local policy.