It depends on heat and moisture. In most cases during the spring summer and fall, it will last at a concentration high enough to kill fleas for about 1 -2 weeks. It will last at a concentration high enough to kill spiders, ants and cockroaches for about 2 -3 weeks. After that, it usually either is washed away by rain or watering, or it is made ineffective by the heat and sun.
Basic rule is reapply when needed. For fleas every 2-3 weeks or until the problem goes away.