The best way to find out when your specific balance will expire is by logging into your Benepass account and looking at the info there. As soon as you log in, you should see a list of all your benefits, separated by whether they are recurring or one-time benefits.
Each benefit should have an expiration date listed underneath the available balance, and different benefits might have different expiration dates.
Since transactions take a few days to go from pending to complete, it's best to spend your balance no later than a few days before it expires to avoid any issues that could cause delays.
Looking for pre-tax benefit expiration? Check here for more info specific to pre-tax benefits like HSAs or FSAs.