I've never had a pup that sleeps all the time, I've often heard it said that it is ok to go out for upto 4 hours as a pup will sleep, but never seen that myself. I can only think they would sleep out of boredom not necessity.
A usual scenario from my experience at 12 weeks, is up and about playing for approx 30 mins, sleep for upto an hour then up again for a mad runabout and some training, then sleep if the mind is more tired for perhaps 1.5 hours, I can't recall a young pup sleeping continuously during the daytime for much longer than that. :-)
As they get older they sleep for longer, but then they are up for longer too. :-)
When my pup was 12 weeks i cant remember much-i was knackered. As i have not got children i didnt quite realise what a broken nights sleep can do to you!! :-)
Like the rest of you i found he had lots of little naps but was never asleep for a very long time.
To be honest i think they sleep more as they get older, but then they also play for longer too. I would think breed probably makes a difference aswell.