待っている人がいるならば、やはり2件ぐらいが目安だと思う。それでも足りない場合は再度後ろに並べばいい。20分もATMを使用した後の並んでる列を見よ。中には罵倒を飛ばす人も中にはいる。そんなことをされたら、その日の気分は台無しではないか?Ddroped by the post office to get some bread a few days ago. There has't  been so crowded. But  I waited for 20 minute then because of the Lady who did transfer, make a withdrawal, update a book etc...Give me a break. I guessd there is a invisible rule. 

I think if there would be waiter, you should have done 2 ~3 things only as a guide.  If you want to use it more, you get on a line again. Look back on behind you.. didnt you feel the weight of waiter angry eyes?? If I were you, I would be guilty and sorry?