Multiply your original estimate by 3, works most of the time
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So: 2 hours -> 4 days, 1 week -> 2 months, etc.
(I'm not sure where this turned up, but it's a long time ago, going on three decades.)
The other option is to carefully track tasks, relevant dimensions, estimates, and actual performance, and see if there's any prediction modelling which can be derived from that. Problem is that this a classic instance of modelling in which the model itself affects the domain (as with economics and sociology, contrasted with astronomy and meterology where this isn't the case), such that estimates incorporating past performance data will incorporate the fact that the prediction model is being used.