I know that you can't walk a puppy "too far" or "too long" but how far is far and how long is long? [

] My lab is almost 12 wks now, and I'm just wondering what the best way to gage our exercise is. And, since I live in a condo, we do more walks than outside yard playing. Is the exercise limit a total, or per session?
I read and re-read my books, and I've asked the vet tech that lives in my complex, and I've seached the net. But the typical answer is "not too far" or "not too long".
Is there an exercise progression guideline like there is for the ability for the pup to hold "it". (ie: months old +1).