My survey calibration chart.
5 minute surveys, 8-10 minutes
10 minute surveys, 12-15 minutes
15 minute surveys 20-22 minutes
20 minute surveys, 25-30 minutes
30 minutes surveys, 40-45 minutes
Anything longer than 40 minutes, pack a lunch.
My rule of thumb is, I don't do any surveys with time listed over 20 minutes. If a survey does not list a time or a progress meter, I do not take it.