I would say 45-50 so you can get to 100 which is the max, but thats if you have both expansions, if it's just the base game then like Pug said, 34-37.
Preferably level 34 - 37 as suggested, but the enemies seem to scale up depending on your stating level. I restarted on level 54 and the drowners at the beginning of the new game+ were level 50.