It depends on a number of things, mostly three: the cause for failover, the version of DART or VNX OE you're running, and the amount of mounted FileSystems and Checkpoints. Ask your Mid-Tier specialist to refer to the "Celerra dn VNX File Data Mover Failover - Theory of Operations and Estimated Timing"
Is there a way to know how long will it take to recover from a DataMover failure or an user-initiated event?
How long does it take for a DataMover to failover?