It usually takes 30 to 60 minutes for sildenafil to work for erectile dysfunction. You can take it up to 4 hours before you want to have sex. Taking sildenafil alone will not cause an erection. You need to be sexually excited for it to work. Sildenafil tablets for erectile dysfunction come in different strengths ranging from 25mg to 100mg. The usual dose is 50mg when you need it. Do not take it more than once a day. The dose can be increased to 100mg or decreased to 25mg, depending on its effect.