on Awstates You have Unique visitors and Number of visits, for how long is it consider a number of visits, not unique visit. For example a person comes on the website today, he leaves and comes back 10 months later, will Awstate consider that Unique visitors or Number of visits? For how long will it know for? Is it until the IP changes, which is every three months?
30 days (upto, for the current month only) which is the snapshot (by default) AW Stats produces stats for. And it is based on IP address, AW doesn't (from memory) log session_id()'s.
yes. this is because since AW Stats produces monthly reports. A visitor who first visits your site on 30 September and revisits on October 1 is classed as unqiue in both September's and October's report