Following situation: You need to do something at the post office – pick up a parcel, an inquiry, whatever. Coming into the post office, you take a ticket for the waiting line. After taking it, you realise that there are over 20, 30 or more tickets before you. How to know if it is worth staying or doing something else in the meantime without losing you place in line by coming back too late?
My experience: Multiply the number of tickets before yours by 3 (assumed minutes spent per person on the counter) divided by the number of open counters.
Example: You got number 184, current numbers called to the counter are 121, 122, 123; open counters 4. Ergo: 60 people times 3 minutes = 180 / 4 = 45 minutes waiting; i.e. enough time to do half an hour shopping and be back in time.
Warning: 60 people in line and just one open counter means that you might want to try your luck another day.