The Duration of maelstrom stacks is refreshed whenever you change Timelines in The Temporal Sanctum. This allows you to infinitely stack maelstrom, i am not sure if the damage also scales or if its just a visual Bug. Also after ~50 stacks all enemies aswell as all minions start lagging and teleporting around.
To recreate all you need to do is press maelstrom and then switch Timelines before it runs out, this can then be repeated infinitely
update: After swapping my Druid to a build that scales Maelstrom i can confirm that it is NOT a visual bug and that the Maelstrom stacks do deal damage.