During a benefit year, there aren't time limits for submitting eligible medical expenses. We recommend submitting as soon as you have the documentation because you can't be reimbursed until your expense is verified and approved. There is only a time limit in place when a benefit year has ended.
At the end of the benefit year, you'll have a 90-day run-out period, which acts as a grace period, allowing you to submit any expenses from the previous benefit year that you might've forgotten. This means you'll have until the end of March to submit for the previous benefit year before it closes out.