It's obviously different for everyone. I've had the spider(for GS and later bow) spawn after 8 hours twice in a row, and I've had KoH spawn EXACTLY on the first PH kill 4 times in a row.

At most, you could probably spend several times longer than getting emp items just camping NMs. At least it can take what, 12 hours? Personally took me 3x as long camping the NMs as it did getting 50gems and 50lanterns.