The top of beginner is having passed L1 elements or free, isn't it? I don't know the actual average, but a couple of years wouldn't be at all unusual for a once-a-week skater. They'll be spending some considerable time working on the field moves test first in most cases and then lots of people wait until both free and elements are ready to test at the same time. Then there's waiting for test dates to come up!
But anyway, it doesn't really matter. The right amount of time to compete as a beginner is... as long as it takes.
There'll be kids who've tried to test up and failed on one element. There'll be kids who've spent several months out with injury/illness. There'll be kids whose family circumstances mean they only get a lesson one week in three. There'll be kids already doing 8 hours a week, with intensive coaching. And yes, there might be kids overstaying in the level to win. But then you'll find that the skater you just watched be *surely* too good for beginners actually only got on the ice for the first time 6 months ago.
The best thing is for your skater to choose her own goal for the event that doesn't relate to anyone else (eg. 'get my spin called' or 'get positive grade of execution marks on an element') and think of placement as an added bonus.