I'm going to have to go down the list of longest manga and see what I've read.
|I read about 8 volumes from the local library|
and then I got really, really bored.
Okay, at 45 volumes, Oh My Goddess seems to be the longest series I've read a good portion of, but I didn't actually like it, so...let's keep looking?
Whelp, the list I found only gives me the top 100 longest. So, I guess I'll go with...Bleach? Although I've only ever read about 5 volumes of it, so I don't really know. So, my final answer is: I have no idea!
Are there any long-running series that you'd recommend?