Originally Posted by coolnout
You can browse completed tourneys to get an idea of their runtime.
^^this is the best way. However, if your chosen tourney is an infrequent one and you can`t find a similar completed example in the lobby, a rough rule of thumb is that the number of players remaining in a standard online MTT halves about every hour.
Therefore, a 100 man tourney will take ~3 hours to reach the FT, and a 500 man will take ~5 hours. This obviously varies to some extent from site to site (and even from day to day) but gives a reasonable approximation.