Some quick terminology. In L3 MM redundnacy, there is a Primary and Secondary. If Primary goes down, secondary will go active after 15min (as noted by phevi0). However, the Secondary MM will be read only (which means you will not be able to make config changes on the MCs reporting up to the Secondary MM). This is to prevent split brain MM in case the Primaries are down to one set of controllers but not to another in a different region. You can manually promote the secondaries to primary if you know the primaries are down hard and you need to make config changes, but that means you need to mark the down primaries as secondaries when they come back up.
If the primaries are down for a short period of time (day or two) and you leave the Secondary MMs as is, then when the primaries come back the MCs will go back.