I realize the infamous t-mag article slams usnic acid pretty bad. Yet it does contain some useful information. (Take it with a grain of salt.) And I quote:
"...Plasma usnic acid levels showed a triexponential elimination with a mean +/- SD terminal half-life of 10.7 +/- 4.6 hr. Peak plasma level (cmax) of 32.5 +/- 6.8 micrograms/ml was achieved in 12.2 +/- 3.8 hr (tmax). Mean absolute bioavailability of usnic acid following oral administration was 77.8%.9 In other words, the body absorbs it rather well, and it hangs around for a long time."