Stromectol Prewscribed for resistant head lice. How long does the drug stay in my system? To kill?

Please see the abstract below.

Stromectal (ivermectin) is an anthelmintic agent structurally similar
to the macrolide antibiotics but without antibacterial
activity. An oral dose of 200 micrograms/kg,
repeated in 10 days, has been shown to be effective
against head lice. If ivermectin gets past the bloodbrain
barrier, it blocks essential neural transmission;
young children may be at higher risk for this adverse
drug reaction. Therefore, ivermectin should not be
used for children who weigh less than 15 kg. It is
not currently approved by the FDA as a pediculicide.

http://aappolicy.aappublications.org/cgi/reprint/pediatrics;110/3/638.pdf

http://www.drugs.com/mtm/stromectol.html