Thanks for the report Daniel. Invites should definitely be good for 14 days, but our invitation code was creating them with 10 day expirations for some reason. The first invitation e-mail text has 14 hard coded in it, so that was still right. However, the follow-up e-mail actually calculates the days remaining on the invitation, so it was using the 10 day expiration we store in the database.
Sorry for the bug. You won't ever get to see it work correctly :( but it will be fixed on our next push.