Based on current agi amounts, you'd see a 2-3% gain in Rip damage for the duration for each stack of the buff, so say waiting for two stacks, you'd see 4-6% damage increase. However, you'd be sacrificing a Rip tick which amounts to a 9% damage decrease over the duration (if we consider the first "non"-tick to be a tick and the following 10 ticks after GoBL extensions).
The percentages are based on my current gear (which isn't good and serves as the lower end), rough numbers from Mihir's 410 sample in Mew and the reported AP scaling of Rip in MMO's database.