The MicroMasters certificate itself does not have an expiration date and you do not need to renew it or earn credits to "maintain it" over the years.
NOTE: RIT can decide to no longer offer the MicroMasters program for new enrollees, but your certificate link that you already earned will stay valid.
The ability to obtain credit towards a university degree using a MicroMasters credential is decided by RIT, not edX. At the current time, many RIT programs use a time limit of seven (7) years for when credit will "expire," so a MicroMasters might not be able to be used for credit towards a graduate-level degree after seven years. RIT may reevaluate this and establish different time limits in the future. Also if someone earned the MicroMasters certificate many years ago, RIT may review this on a case-by-case basis to ensure what the applicant learned in the MicroMasters program is relevant to current university degrees. If you are interested in pursuing study at RIT, we recommend applying as soon as is feasible after earning the MicroMasters certificate.