First try to understand that you need to post constructive and constructive means you involve in a discussion or share something which is new and relevant to the topic. If there is anything of your interest, you may post  3-4 posts in an hour too and at times you will not post for period of 12 - 15 hrs or even a day.  

By the way, constructive posting is not a 9 - 5 Job where you post twice every hour.  
Believe me while posting if you have something at the back of your mind that you have to complete 18-20 posts, no matter what happens, you will end up spamming.