you should respect the time indicated, because a longer exposure to the componente of the whitening system can lead to a greater tooth sensitivity than it would normaly do. also it could irritate your gums. remember that the ingredients of these products are hidrogen peroxide or carbamide peroxide (really strong stuff) and leaving them too much time on your teeth can cause unwanted effects. hope info helps
P.D carfull with coffee, tea or other food or drinks with strong colorants while using a whitening system