TWC has been upgrading firmware on modems about once a year if newer firmware is available for the past 2-3 years at least, mostly since the DOCSIS 3 configs on the gear started getting turned up.
Most of the Moto's were updated about the March/April of last year that I noticed and it was done in waves based on area and model, which is what it seems they're doing again. They may update faster if some critical problem pops up and firmware with a fix is available.
Comcast does firmware updates about twice a year. Charter about once a year too. Cox about once or twice a year, I think. All lag at least a 2-6 months behind the manufacturer release of the firmware so they can do their own testing. There always seems to be somebody bitching about their provider's upgrade schedule in one of the various cable provider forums when they don't have the absolute latest version of firmware, even when they have no clue what changes were made in it.
Moto releases new firmware every 2-3 months for the 61xx series, 220.127.116.11 was released 2 months ago, so a new version will probably be out next month. Maybe about every 6-12 months for the 51xx series, last one for the 5101 was in April 2012. I've never seen any provider even try to keep up with every release.--
Two is one, one is none. If it's important, have a back up... 99.999% availability just isn't enough sometimes.