brotherhood_of_LAN - 4:36 pm on May 16, 2013 (gmt 0)
I'd just run it every minute, but be sure that emails don't get sent out more than once, i.e. flag them as "being sent".
For Windows, not sure but if there's a control panel with your hosting, have a look at it. Windows does have a cron-type service.
If you hit a limitation of your hosting, then there's not too much you can do about that.