It lasts about 30min but I've found that it depends on how thick you apply it. I'm liberal and repeat a few times before starting treatment so the effect sinks-in below the surface a bit.
I've been going for 90min appointments where my technician scatters hair removal across the face during a session because I've found that I swell quite a bit if she stays in one general area for awhile. In addition to the initial application, while she's doing the left side, I'll apply the cream on the right so it's still numb when she moves to that side. As my beard gets more sparse I'll reduce the appointment times because inevitably she'll be concentrating on one section during the entire session.
Hope that makes sense.
Also, FWIW, my technician once used her numbing cream on me (not sure which one) and I found that it wasn't nearly as effective as EMLA.
LOL, honestly I don't care how much the stuff costs and how much I have to use if it takes just a bit of the edge off