Sorry, I didn't took the time to read the whole thread, but it's mostly a personal answer.

The last addon I coded took me 5 months to code (and it's not already finished, I'll had some more things in the next year), but I had to go to school, and also, I didn't spend all my free time coding it. My addon was quite a big thing (it's now 1250 lines and 52k of codes), and also, it was hard as I had to respect multiple protocols, and also, I had to support a lot of connections (it's a MSN Messenger client for mIRC which I called MSNMIRC, for information, a link to my website is now available in the "More Info" section on this website.. thanks to Tjerk, it was a hard work, but it's worth it I think). So I think you could get a script done in 6 months if you code when you're bored, you make an all-around script with lots of features and you don't make it too difficult. The one I'm coding now with a friend, I'm giving me a year before the release, as I'm coding something quite big with a new theme engine supporting emoticons and a lot of other features.

If you think you can support coding a script for several months, go ahead, you'll be proud when you'll release it and get feedbacks from people who downloaded it. Just one tip: make it customizable for the user.. everybody likes to have a script work the way they want!