Hi, I finally asked my husband (who goes to way more meets than I do) and he explained to me that in our experience (we are region 1) there are three types of warm up policies and which is used depends on the meet. But he forgets the official names for them.
Typical at most meets is "Warm up one, compete one." This involves an initial 20 minute open stretch period, then march in, then 10 minutes warm up time directly before competing each event. 10 minutes no matter how many kids are in each group. (DS said that sometimes with very large groups warm up time is increased 15 but husband disagrees.) But, when there is an extra large group and some kids were clearly shorted on warmup time, the event judge might allow a very little bit of extra warmup time for those kids. This is entirely up to the judge and rarely happens.
Other most common (but much less common) format is "Warm up all, Compete all." In that case there is an initial long warm up period where each team spends time warming up on each event, but once the competition begins, each event is competed one after the other bang bang bang.- no warm up time directly before going on the event. None. For older guys (optionals, or maybe only 10s?) they get a "touch" (I presume to make sure the equipment is set correctly) but do not run routines or do anything at all like a typical warmup. So this is a case where a gymnast would have to be capable of competing their vault without vaulting or running timers or anything at all right before it. If they are competing vault later in rotation, it may have been quite a while since they warmed up their vault.
Lastly and least common is "Warm up two, compete two." This is for very large meets where there are two flights going at the same time, and according to my husband usually only happens in the compulsory levels with huge amount of kids competing.