Homeopathic, indeed. http://www.unboundmedicine.com/harrietlane/ub/citation/15135... states about 2 microgram/cm^2/h absorption of caffeine.

http://en.wikipedia.org/wiki/Caffeine gives about 100 milligram per cup of coffee.

Assuming 100 cm^2 exposure area, you would need a milligram per cm^2 for a "cup of coffee equivalent". That's 500 hours ("three weeks")

And that is assuming that this spray is equivalent to whatever van de Sandt er al used to get caffeine on the skin (I would guess they applied way more than a spray would apply)

On the other hand, high pressure spraying (better called jet injection; see http://en.wikipedia.org/wiki/Jet_injector) might work. I think that would require FDA approval, though.