the only thing i've ever seen on them is if it's worn or damaged to replace it. i've never seen any kind of spec for play or anything. it's pretty much a judgement call i guess. i treat it like the cushions in a clutch basket. if there is play in them by hand you know it's going to be worse under stress. for all they cost i just replace them while i have it apart.
a guy that works for me has a shadow 750 and his is worn pretty bad, like at least 1/4" of play in it. i told him to take it back to the shop because he just bought the bike and they told him that was normal, it's supposed to be like that.
i'd say that is pretty excessive & advised him not to go back to that shop for repairs.