Simply by using a boolean. Just create one in your character/controller BP and then do something like this:
Setting it back to false in OnCompleted is not the best way to do it. So you can just add a delay after the initial execute and set it back to false:
Still, this is also not the best way to do it but most beginner friendly way.
Best way to do it could be like this:
But for this to work, you need to setup Notify Begin/Notify End on your montage.
1 is Notify Begin, 2 is Notify End. Just place it to your liking.
I hope this is helpful, wish the best.