It's totally arbitrary, but I think amongst the general cycling populace (not just hard-core high level racers, but also not really counting folks who ride their bike less than once per week), I like the following:

- Sub 1hr 40k solo time trial with full aero gear. If you can do a sub 1hr 40k solo without any aero gear, you're better than just 'good'. Not great, but for sure you'll be doing some damage at the local races. If a triathlete can do a sub 1hr 40k even with aero gear as a standalone bike race, they're typically fast enough to place or win their age group in a local olympic-distance triathlon in the most competitive age group.

- Cat 3 or above in CA also shows that you're a pretty solid cyclist. Shows that you can handle your bike pretty well as well.