Its not so much about waiting for her to be ready, as it is planning for when you want the calf. Most decide this based on expected weather that time of year, and what market they will hit when that calf is grown.
Example I dont want to be calving during the coldest months, and also have the calves a month or so old when grass is ready, so I aim for late march calves. They then eat great grass until mid sept or so, and are ready to wean as my grass is ending for the season. Putting the bull out in june, gives the cows time to recoup and be ready to breed again to calve again the following year at about the same time.
If you want her bred as soon as possible, just put her with the bull and she'll decide.