I have always heard 10 days is the general length of time for most auction bugs to have run their course, but that 3 weeks is safer. I think 2-3 months would be great, if given the opportunity... But unnecessary.
My BO, before I moved in, adopted a kill buyer horse from across the country knowing absolutely none of the horse's history. I can't remember if she said the mare was quarantined at all during or before the shipping trip over, but she did not quarantine more than 24 hours when the mare arrived. She got lucky and ended up with a healthy (though underweight), amazingly friendly and trained sweet little girl. I hope the same goes for these two!