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 best time to take sildenafil is about 1 hour before sexual activity, but you can take the medication any time from 4 hours to 30 minutes before sexual activity. Sildenafil usually should not be taken more than once every 24 hours.