My favorite poker answer is "it depends". However, clues are available in the structure sheet of the tourny. For example, the 200 structure sheet at the Orleans open says the rebuy period ends about 3 hours and 10 minutes after the start. Since most who bust rebuy, this is when the tourny REALLY starts. If you wait until this point to enter, you will have 10 bb, and be in an all in or fold situation, and people start to drop like flies. It is reasonable to expect the tourny to be over by level 19, when the bb is your starting stack...if it goes beyond that point, and you are lucky enough to be there, you probably won't care much how long the final table takes...and there will likely be some form of chop anyway.

Bottom line: If you are looking for a round figure, about 6 hours ought to do it.@ezryder17