Hi there,
My thoughts are that in reality its going to be hard to enforce any kind of rule in the game that is able to effectivly stop this from happening. or to even lessen it.
that being said… if we are looking for a more accurate sense of when it becomes rude then for me it would be:
1 Hour+ game// half the main time, plus buyomi.
30-50 mins game// Half time plus buyomi.
10-20mins game// Five mins plus buyomi
5-10 mins game// Three mins pluss buyomi
Anything below this, time out is acceptable i think. (With the exception of longer buyomi, of which if there are five on min buyomi, then after four i generally know they are most certainly not coming back)
I hope this helps.