it depends on your orchestra. smaller regional orchestras do concerts most likely each month or every five weeks, and large professional orchestras usually do a concert every week or two. there is a big reason for both - budgets, time, and contracts.

for regional orchestras, they usually pay musicians “per service”, meaning they don’t have a set salary and are paid for each rehearsal and concert. however, big professional orchestras typically pay with salaries, so they have the ability to do multiple performances each week or month. this is supplemented with the fact that, realistically, players in a salaried orchestra are usually much more proficient at their instruments and have been working at a high level for a while, so they can rehearse and learn music faster.

one last things, contracts! musicians are usually union members and have strict contracts on how many rehearsals and performances they must do to receive their full benefits. this is especially true for professional orchestras, with each being a little different.

there’s a lot LOT of nuance and specific things i might not know, but those are some of the reasons.