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In the case of the North, it should take not that far off- in about three months, Robb Stark was only able to gather about 20,000. As is clear from the more recent books, as well as the sheer amount that Torrhen Stark was able to raise during Aegon's Conquest, there are clearly far more troops left in the North. On the other hand, the Lannisters seem to be able to raise almost their full levy in a short amount of time. So basically, there cannot be a universal rule.