They process bids every month and if there is an opening and your seniority can hold it, you will be awarded it. You could change bases every month if you wanted to and had the seniority. But after IOE there are only a couple bases that you can hold then anyway.
For reference, I was in 3 different bases in 4 months during and after IOE. But keep in mind that you might be finishing up a trip in one base on the 31st, and then have another trip in the new base 2 days later.