No more then 6 hours a day if you want to be safe. Switch worlds, and go to a different spot. Do it on a pvp world and talk to other alchers occasionally.
id say just use pc boats on a non pc world..although people say its 'unsafe' becuase mods supposidly go there, my friend auto alches 90-94 mage on his zerker there. just dont do it more than 4-5 hrs a day