I've gotten my breedjects back a number of times, sometimes fairly quickly. It tends to happen more these days than it used to, as I find that Wonder Trade is quieter than it once was, and it's harder to find a trade partners. If I'm playing at a weird time for players in multiple time zones, I can easily get by breedjects back within a few trades. It's rare, but when it happens, I often don't have the heart to send them back out again.
I do wonder what the algorithm for determining wonder trade matches is, though, as Fannie pointed out. I've never spent enough time tracking my own wonder trade partners to look for patterns, though, and I don't know how obvious the patterns would be, anyway.