I suppose it's possible the poster's IP is stored in the post metadata itself, along with the date, username, and so forth. Don't know if that's how Reddit works but in that case it'll be retained forever.

As for the reason for 7 years worth of data, I think that's because the firm wants to go back and search for infringing acitivity on all those IP addresses due to dynamic IP updates and so forth. If the user is still active on Reddit the more recent IP addresses probably are enough to identify him if a warrant is served to the ISP. The ISPs I know only retain a link from a dynamic IP to a subscriber for one to two years max.