it all depends if your willing to do pushups daily for the rest of your life...If yes then try doing a minimum of 4-5 sets of 20 pushups every 7 hours for atleast 2 weeks. Now that your stronger try doing 6-7 sets of pushups for more than 3 weeks iknow it sounds alot but remember all this painfull work leads to a better future with your body!
Depending on if you are a beginner, you will need to do fewer so not to get to sore. Just work your way up. Do 2 to 3 sets of 15 to 20 push ups at first. Then when you are feeling stronger, 3 to 4 sets of 20 to 30 push ups daily.You can find more information here: http://www.ehow.com/facts_4968913_many-pushups-should-do-day.html