Donald,
There is no rule of thumb as the amount of money that needs to be set aside is based off of common area that needs to be maintained. If there were two identical developments but one was required to maintain the roads and in the other one the city maintained the roads, the reserve amounts would be drastically different.
Your Association should have a reserve study done. There are firms that will do them for a fee or the Association can do one themselves. Here is a link to a thread about reserves from this forum:
http://www.hoatalk.com/Forum/tabid/55/forumid/1/postid/103517/view/topic/Default.aspx Tim