Instead of 7 calendar days, how about something like:
6 login days (don't penalize people who don't play a lot), or
5 days of going into dungeons (don't penalize people who log in to socialize or alchemize), or
4 uses, or
3 days of use (benefit to using it more than once on a given day)
How about... They don't ever expire. I'll probably never go back to Haven otherwise.