Asterius and his accompanying berserking bisons can take out your entire party if you don't spot him coming. Be careful about aggroing him if there are monks near him as it can take a while to take the mob down.
In normal mode, it takes him 8-10 minutes to patrol his route.
If you are hunting him ( In Normal Mode ), go outside Olafstead, get the bounty from Olgana Beartamer, then immediately head to the shrine that Inga Caveborn is standing next too. You should be able to catch him coming up the hill NW of the shrine.
If you are hunting him ( In HM ), go outside Olafstead, get the bounty from Olgana Beartamer, walk a little bit to the west, and wait for him, he will come in roughly 1 minute. Remember: This only works in Hard Mode.