I've never used epoxy on tall poles (20' plus) Seems like a lot of damn drilling. I would think you'd have to go in at leats 24" as the stock bolts are usually 30".
What I do (sometimes) is take out enough concrete to get ahold of a couple inches of good clean thread and install rod couplings/all thread. I don't know if it's engineer approved but with 4, 3/4" bolts, spaced about 6" apart, tightened all the way into a rod coupling on both ends, the chance of a failure seems miniscule.
Don't forget to deal with the water issue that caused the bolts to rust in the first place or someone will be back doing it again. Last year, in a water retention area, I simply raised the base about 24" (with threaded rod and couplings) to get it out of the flood water level.