That depends on the package maintainers.
For e.g. the *buntus packages are maintained by Kubuntu and the KDE (sorry for mentioning the K-"word" again
) guys at Debian. For Trusty the gtk(2
)-engines-oxygen were never updated which also effects all distros based thereon, those will be around until 2019. Likeliness that it will be updated? Very low. So, even if the change lands in WIly (15.10), the bug will still exist for a while. Depending on how well the next LTS will be accepted, 16.04 could bring a change. Debian Wheezy had it's last change in 2012, still to early to say something Jessie, was just released.
It's not like: now that the bug is fixed, everything changes quickly. That's only true for some but by far not all distros.