Oh gosh.
My first long running comic took me about 2 months to plan and execute.
My buffer took roughly 2 weeks to make and I made about 30 pages in that amount of time (and they didn't look all that good according to my very honest friends xD I had to go back and fix it later).
The second comic I made, a long runner as well, took me about 10-11 years.
Yeah. I had an idea that I tried, tried and tried so many times to execute but I was never quite happy with it.
I think I re-booted that story about 4-5 times, all with different ideas and angles but it still wasn't interesting to me.
I had to stop at 75 pages, and I just couldn't get myself to work on it anymore. It felt so boring and I just didn't feel like this was something I wanted to work on at all.
It wasn't until I made a few key changes and added a few more things to spice it all up that I felt like: OH MAN I CAN'T WAIT UNTIL I CAN GET MYSELF ONTO WORKING ON THIS YESTERDAY.
When that finally kicked in, it took me about 3 months until I started to thumbnail and sketch and finalize the first chapter.
But we all work at different paces, so I might be considered a fast artist? IDK, but I've been told that xD
As @Shanny8 said: Only you can determine whether or not you're ready!