I've wondered about this too. Obviously depending on skill a player could multi-table any stake. I think OP's question is where is the end of successful multi-tabling playing ABC auto-pilot poker? You can get away with it at 2NL and 5NL. I've heard one could have success at 10NL doing it. From everything I've read here, it seems as if the line is drawn at 50NL. That is where the multi-level thinking comes into play (i guess).

If a player could sustain a 6BB/100 winrate at 25NL and get in 10K hands a week, that's $300 a week. That's a nice part time job auto-piloting 10+ tables for ~3 hours a day.