Just as an addendum, there is more to consider about the duration of DEET repellent than just its concentration percentage. 3M Ultrathon and Sawyer Premium use a “controlled-release” mixture that releases DEET over a long period of time, so even though these use a “lower” concentration of 20–33%, they last much longer than regular DEET repellents.

Of course, you will also have to consider the environment you put yourself in when considering the duration of the repellent, for example, sweating, swimming, and rain.