For the current ranges of the Cunningham project (August 2016), the largest number is 3^850 = 1348 bits. For numbers of that magnitude, ECM can quickly find factors of 64 bits or less: between 5 seconds to 2 minutes depending on software, hardware, and luck.
This serves as a nice threshold for factors which do not need to be carefully preserved because they can quickly be recalculated.
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