There is no absolute maximum. It's analog, so to fit more on a side the dynamics and volume will have to be reduced. The theoretical maximum would have to be the stylus tip upper width x the maximum distance from edge to label. I'll leave the math to you :P
I do have this Rhythm Of Time 7" though which has quite a long b-side. Only possible because of the soft music I guess.
the reason I asked that is because I am considering pressing a record, and I want to see my options.. I want to make a 4 tracks EP, and each tracks are about 7 mins.. so I am looking at about 14mins per side. Obviously 12" wouldn't be a problem, but I wondering about 10"... I would be interested in this format (reminds me of the 100% Acidiferous and Pounding Grooves records ;)