Sometimes it just depends on what the pup will tolerate, with Tango I had started with 10-15 minute sessions, about 5 times daily when she was 8 1/2 weeks old and gradually, as her attention span lengthened and she became very excited for her training sessions I'd train her for about 20-35 minutes 5 times daily because she loved it (This was around 14 weeks). I don't think I ever went any higher than 35mins though. I'm not sure what the average is though, but this is what worked for Tango.