Spread the dose out to twice daily...and there is a difference between half life and half dose timeframes.
For instance, the half dose is about 12-18 hours (wide margin since skins types and conditions vary greatly from person to person) and is the time when half the dose applied is abosrbed.
From what is absorbed the half life of the PH's are the same for most other non-alkylated compounds: about 6-8 hours. The difference, though, is that topicals are slow release so there is a constant flux of PH's into your system.
Chemo