I keep cards and letters forever, unless they have no messages in them (except my mother's because that's a different kettle of fish, she loves cards but oddly doesn't really fill them out, I can handle it because she's extremely thoughtful over her greeting card choices). They take up a shoebox if that, I just keep them in a specific location in my keepsakes.
I do go through them when I have a feeling and giggle over tons of things.
I saved a bunch of things from grade school as well, I've always been organized (I used to file all my school work in a filing cabinet in my closet until I was in 7th grade).
I reconnected with my best friend from elementary school a couple year ago. She's amazing, we're now extremely close. I found an old drawing she gave me when we met in 2nd grade saying "Hi Rebecca, you are a good friend, you are nice to me." and a picture of us together. (With some killer spelling choices, haha). So I got to fish that out and share with her. I have my 5th grade journal still. I read it to her one day because it was about our "adventure" to get her supplies for her monthly cycle she had just started...she looked at me and said "I feel bad..." "Why though, it's funny!" "No. I feel bad because I faked that, I didn't remember that I had told you too! I was jealous of my cousin and decided I'd tell everyone that I was having mine too." AHAHAHAHA, the things that we have tucked away in drawers.