I think it's all about the quality and how advanced it is. I spend more than 3 months on my first plugin. Because it was my first plugin, but also it's very advanced with a lot of features. Since the release I am working on it like forever.
While that makes sense, you've yet to release even one plugin on CC even though you've got a couple of pretty popular items / files / products (neither of which I'd consider to actually be a "plugin" in the sense it was likely intended by the OP).
If you were to ever finally finish / release your 1st WP plugin for either of them though, they'd probably be instantly successful as well (if necessary quality and features were there) even though I personally have no need of such things now.
The comments of both items have been peppered with requests for a plugin for about 2 years now. The reply has typically been that a plugin is being developed. While not unheard of, that's a long dev cycle. I imagine many could've learned to program and have made their own quality plugin in that time too. Lol.
Having written several actual plugins myself, it has taken anywhere from 1 day to a month typically with the exception going outside that range for me. Drupal ones take the longest with WP ones usually being finished the quickest which is likely due to my (lack of) experience with both platforms.