On Casseopeia, who gets more damage per AP per second than any champ I can think of off the top of my head, 125 AP from Wooglet's gives her 121 damage per second. BFT, only considering the passive and AP, gives her that much dps on a target with at least ((125*0.97)-(70*0.97))/(0.05/3)=3186 health.
Simplifying down, (ignoring stacks, keeping the DoT ticking, and additional sources of AP going with the 25% bonus, because otherwise math becomes literally impossible) the amount of health a target needs to make BFT better than Wooglet's is 3300*(Damage per AP per second)
Annie, for instance, assuming she procs Molten Shield on average once per second (which is pretty generous), has a 0.55 ratio, making the target's health needed 1815. Ignoring Molten Shield, it's 1155.
And, obviously, BFT is much better than Wooglet's on non-mages. BFT is better on Ashe for targets with 41.25 health.