As far as Amber knows, the amount of time is...fluid, just like much of what happens in the supernatural world. The natural energy barrier that build up depend as much on intent as much as the aura of the individuals. The invitation will likely fade over time, or fade if there is some sort of significant change to the individual.
Harry pokes his head back out. "Good, looks like she's gone." he grunts. "Now, Amber and I will be performing the umm...magical equivalent of a chemical spectral-analysys. But, we don't have everything here that we need. In order to facilitate things, I'd appreciate your help in acquiring these items. In the meantime I'll start drawing the necessary circles." He hands you guys a list.
For Brando and Grim, harry wants you to use your Contacts and your Resources to quickly acquire certain materials that aren't easily found at your local walmart. Amber can choose to help with material gathering OR choose to help out with drawing the necessary magic circles.
Brando and Grim should both roll their Resources and Contacts. What you guys get will help determine what kind of bonuses Harry and Amber get in their Thaumaturgy in this case. Amber can also do the same, OR if she feels that her skills at magic circles are better, she can attempt to give bonuses by rolling a Lore roll instead.