when is it better to upgrade my F5 box to a new version of TMOS?

For a major release version I generally wait until several releases have occurred. For instance, with 12.x, we didn't put it into general production until it was well into the 12.1.x branch, around 12.1.2.

As Jie mentioned for hotfixes and minor updates inside of a major branch, we generally apply these within 6 weeks of release. Depending on the severity of any vulnerabilities that may be fixed by them, we might do it more quickly.