Traditionally, iOS has retained your entire conversation history in the Messages database,so if you send a lot of pictures and videos via iMessage, this can eat up quite a bit of space on your iPhone over time. Prior to iOS 8, the only real way to deal with this was to remove an entire conversation, basically starting over, or to manually scroll back through your Messages history and remove individual items one-by-one. Fortunately, in iOS 8 there’s a slightly easier way.
If you go into the Messages section in your Settings app, you’ll see a new option, “Keep Messages.” This setting allows you to tell to iOS to automatically purge anything in your Messages conversations older than the specified time frame. Unfortunately, the options are limited to either “Forever” (the behavior in past iOS versions), 30 days, or 1 year, but even 30 days is still far better than having to go through and delete an entire conversation if you’re running low on space. Keep in mind that enabling this option will immediately purge everything older than the specified time frame, and continue to do so as a rolling thirty-day or one-year window, so if you want to keep your old conversations backed up somewhere you’ll want to look at archiving them using a third-party Mac or PC app before you take the plunge.