You probably have to take into account how long the user has been active. A user who logged in once and never again probably doesn't even remember signing up, while for a user who has player for a year, they are unlikely to have forgotten about it after a month.
Web marketing people will probably tell you that a targeted email list is one of the most valuable things you can have, and that you should keep investing in your inactive users because it takes a lot less effort to keep them playing than it takes to get new users.
I played EVE Online for a month once, and I still get newsletters and crap from them. But they send me valuable "come back" discounts, so I can see the value. The point is that if you're making money from your game, then chances are it'll never be worth it to completely deactivate an account because there's always the chance that a user will come back and spend some more.
I think 3 months would be a good minimum wait time before sending a reminder email, and if you really have to delete them for inactivity, 6 months would be a good wait period. But compared to minute cost of keeping inactive users in the database, I don't think it should ever be a necessity to remove users (unless the number of inactive users affects gameplay, but even then you can use an inactivity flag rather than deletion)