Last hour

Unless any playing conditions say otherwise, when there is only one hour left of the time scheduled for the match, the umpire at the bowler's end will signal to the scorers that the next over is the first of a minimum of 20 that have to be completed before the last hour of the match can end, assuming, of course, that the match doesn't finish in a result before that point or that there are no interruptions to play.
If there is an interruption or two - for bad weather perhaps - or one side's innings comes to an end during the last hour, there is a mathematical formula for working out how many overs are deducted from the 20 for each period, based on one over for each three minute time period lost.
So, even though it is always referred to as the last hour, it will often be longer or shorter than an hour!

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