I'm gonna use super baseline and generic math here:

Let's say you have Balance on a monk doing a base of 1000 dps for the fight. Let's say you have that 10% Balance Buff up for 30% of the fight's duration. 10% of 1000 is 100 and 30% of 100 is 30.

You've added 30 DPS to that monk's 1k DPS for the entire fight.

I know it's not the best but it gives an idea I guess.