Hmmm. I wonder if I wrote Don Carman back in 1989.
Loved collecting autographs by mail in 9th and 10th grade. I've still got the binder somewhere here, so I should check on Carman.
The first one I mailed out, Rick Reuschel, was the first one I got back: One week later.
I got an autograph from Terry Kennedy FIVE DAYS after I sent it.
The most memorable one was Mark McGwire. Took just 20 days for him to autograph my card and send it back. I was so happy when I got it, I almost hugged the mailman. Brought the card to the first day of school and my douchenozzle friends were all "No way! You forged that!" because all of them had written McGwire but had yet to get a card back. The next day, I brought in the return envelope, complete with an Oakland stamp, and they shut up.
Can't match the Carman story, but I once got a Frank White autograph 2 1/2 years after I sent the card out. By that point, I wasn't really collecting anymore, but it was still cool.