Rarely. Certainly not more than once a year; if I'm adding a new one to the box, I might leaf through the old ones. But it's a nice sense of joy and continuity, and I can of course know that they're there without having to even open the box. The emotional value like most mementoes varies over time, but trends gradually upwards.

(I do wonder if all the people who say "collect memories, not things" have more vivid, eidetic memories than I do; mine can be a little fragmentary without help.)