My one day maximum is 20,000 ft. (and a few more) which was on the Devil Mountain Double Century in 2014.

http://www.quackcyclists.com/dmd.htm

We started at 5:30 AM on Saturday and I finished with my riding companion (Peg Miller) at 2:15 on Sunday morning. In 2014 I was 64 years old. I still have the photo that Victor (Biker395) took of us at the finish. We really do look done ("put a fork in me, I'm done!" as Peg says).

A couple weekends ago I rode 11,000 ft. in one day (at 67 - my Birthday Ride) but only 90 miles. That was fairly comfortable but took most of the day with a 7:30 AM start and a 4:30 PM finish.

In terms of how often; pretty much every weekend with the occasional exception.

Rick / OCRR