As of memsql 7.0 we bundle an online upgrade tool with the engine, so you can always upgrade with reasonably minimal interruption (the cluster will have less capacity during the upgrade). You need to pass the --online flag to memsql-deploy upgrade to use it (https://docs.memsql.com/v7.1/reference/memsql-tools-reference/memsql-deploy-reference/upgrade/)
If your doing an offline upgrade, then the time to upgrade is usually not that much longer then the time to do a clean cluster restart. The upgrade to MemSQL 7.0 is different then typical as it introduces an entirely new replication protocol so the upgrade needs to modify some file metadata on disk. So the upgrade to 7.0 depends greatly on the speed of your disk and the size of any columnstore tables you have. For columnstore tables with 10s to 100s of TB of data, it could take greater then an hour to upgrade depending on the disk (100K IOPS is going to be a lot faster then if your disk is something like EBS with many 10K IOPS).