Lifetime of Sync State

I am using the SyncFolderItems operation to synchronize my calendar folder.

What is the lifetime of the Synchronization State cookie returned by the SyncFolderItems call?

If I use a 40-day old SyncState string (saved in the DB from an earlier call) in the SyncFolderItems Call, will I get back the 40 days worth of data? Or will that Sync State have expired?

Also, is SyncState string valid between different versions of Exchange server? (2007 and 2010) ?