Here is what Consumer Reports reports has to say on repellents. More is better and most of us do not use enough to be effective.

http://www.consumerreports.org/insect-repellents/mosquito-repellents-that-best-protect-against-zika/

The way I read this is I along with everyone with me have not been using near enough of the active ingredients. We also have not been packing enough. If you want to avoid the many mosquito born diseases you want to use ample DEET early and often.