There are 3 bids per year. 2 are closed and the one in October is a flush bid. If you were recently recalled, you might not be able to hold the 18 day schedule based on your seniority, but after you get out of training, you will be placed on it per the contract. You'll stay on it until:
1- you request the 7/7 before IOE, which may or may not be approved
2- you bid the 7/7 during ANY bid period (you will always be awarded it if you want it.)
3- You get kicked off of it during a flush bid because you don't have the seniority to hold it.