drive line play

[N0tac0p]

OK, not trying to start a war, its just a question, that perhps isn't easily answered.  If i lift the bike, put the bike into 1st and move the tire front to back....and then mark at one of the furthest points on the tire.  How much travel would I expect. 2 inches?  8 inches?   I would think there must be some level that screams, "hey you got a problem". 

then it is up to us to determine if its u-joint, drive bushings, spine wear.....