Here's what it says at http://www.uesp.net/wiki/Skyrim:Lycanthropy:
In werewolf form, the player's damage rating is 20 at level 1. It then increases in five point intervals at every fourth level, beginning at level 11. The maximum damage rating is 70 points at level 45.
- If you have http://www.uesp.net/wiki/Skyrim:Dawnguard installed, the scaling is different and does not follow a specific pattern. The damage increases five points every fifth level starting at level 11, except for levels 16, 21, 41 and 46 where the increase is 10 points. The maximum is 80 points at level 46.
I couldn't find any info on how this interacts with the perk tree (i.e. whether the cap of 80 is the maximum cap that cannot be increased in any way, or whether it still benefits from bestial strength), but my guess is - since rank 4 Bestial strength gives you 100% more damage - that the max damage you can deal as a werewolf would be 160 (with fully upgraded bestial strength at level 46 or higher).