Game commitment levels will vary from one GM to another.  A good rule of thumb is being able to check the site at least three times a week, and if you have an open tag in one of your games, taking the opportunity to reply to it.  Some GMs may have higher expectations for posting and others may only require a minimum of one post per week.  It's important to understand the GM Expectations for each game that you're involved in, these can be found in your game specific forum.