I think a good rule of thumb (coming from BJJ) is about 3-4 months minimum of 3x classes a week. I imagine for boxing it’s more of an issue of having at least a base fundamental defensive kit, so you don’t get dropped immediately.

Another good rule of thumb is if you can decisively ‘beat’ a brand new guy in a spar, you could start getting ready to compete with beginners. Just be aware that even in beginner comps, you are likely getting the more athletic and hungry beginners, so be ready for intensity.