DEET, a pesticide, is contained within many insect repellents and is generally considered safe to use. However, pesticides can be toxic, so it's important to take some precautions. A 5 percent DEET repellent will provide between one and two hours of protection, while 25 percent DEET will last for about five hours. Use only as much DEET as necessary, and do not reapply if you do not need it. Be sure to keep DEET off hands, as it can be toxic to ingest. DEET should also not be used on infants.