approximately how long after the release of a new version of ROS do previous packages come out?
In particular, I’m waiting for the rosserial package for Melodic.
i’m sure with melodic being so new i am going to come across this issue more so im debating switching back (i have downloaded and removed and redownloaded so many times this is the last thing i want to do).
You may see that most people move into a new release a year after it’s released. Others wait even more for the previous release to be end of life… and then the most ‘adventurous people’ move 6months after release.
If you specifically need a package, you can always open an issue on the repo on github. When someone actually needs it, maintainers usually give it a bump (specially if there isn’t any update to do to the code to make it work in the new version). If you can check if it works compiling it from source, and you mention what works or what doesn’t, you’ll be helping also.
First of all, I believe this type of question is better suited for https://answers.ros.org. Discourse is more about announcements and discussion.
Package releases can take arbitrarily long - this is the responsibility of the individual package maintainers. Your best chance is to open an issue at https://github.com/ros-drivers/rosserial/issues and let the maintainers know that there is need for a melodic release. You can also monitor the release status page http://repositories.ros.org/status_page/ros_melodic_default.html for the release status of a particular package. As of now, there is no melodic release of rosserial, so it does not appear on the status page (yet).