The product we use for our GIT is a "residual insecticide", which means it must be ingested, inhaled or absorbed by the pest to be effective.
It is important to remember that the treatment is not a repellent – insects may still enter your property through open doors and windows, in washing etc. However, once they come into contact with a treated surface, they will become affected and die.
The time it takes for the insect to die will depend on its size (ie. a large spider can remain active significantly longer than a mosquito or fly).