Your erection should last for as long as there is sexual stimulation. As Sildenafil only gives you the ability to achieve an erection, rather than producing an erection itself, your erection should last for a normal period of time.
You may find that the amount of stimulation required to achieve an erection is lower than usual and therefore you achieve an erection more easily or it lasts slightly longer than normal.
Sildenafil should not produce an erection that lasts for long periods of time without subsiding. An erection that lasts for more than 4 hours requires immediate medical attention.