At reef temps the lifecycle is 3-5 days with another 2 weeks of infectivity. In theory, 3 weeks should be fine. The recommendation of longer I believe is because of the possibility of Ich or infestations of both, so yes 6 weeks is the recommended fallow
What I didn't know or forgot was that below 60F the cysts go dormant and at 4 months they are still active when temps climb. Could not find how long they can actually last. The infestation can also be spread in an aerosolized form - meaning that a near by QT tank with an airstone or surface agitation could potentially spread marine velvet to the display tank. How scary is it!