Just an hour. But three puffs are enough, so you can simply reapply it well before the hour is up. Best test it out beforehand, though.

[He takes it out of his bag - yes, he did think to bring it along! - and gives it to her.]

I reckon the best way to go about this is for me to lay the groundwork and gain these fuckers' trust, and once I'm in and know where they keep the stolen goods or counterfeits or experimental potions or whatever, I let you in so you can collect the proof that'll allow the police to sack them. How does that sound?