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A good rule of thumb is that the tournament is more or less over when the big blind is equal to 8% of the chips in play.
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Agreed. It may end sooner (based on how the chips are spread between the remaining players, and their styles of play), but probably wouldn't go beyond the 8% level at which the tournament basically becomes an all-in lottery.
Posted this in another thread, but just for grins, here is some actual real life data from the last 12 (home) tournaments I ran. I show the number of players (not really relevant) and the BB% at the time the tournament ended.
<font class="small">Code:</font><hr /><pre>
# players BB% at end
8 4.3%
9 1.3%
9 1.9%
9 1.3%
8 2.9%
8 8.6%
13 7.0%
8 5.7%
11 8.3%
6 2.9%
11 4.2%
6 5.7%
</pre><hr />