Well it's difficult to give you a number since manga/comics don't rely on narrative and all manga have different lengths depending on how popular they are. Also we don't know what writing format you're using. Prose and scripts are quite different and the wordiness of both depends on your style.
If you're writing in prose this may help give you some context to the length of a story.
"In fiction
Classification Word count
Novel 40,000 words or over
Novella 17,500 to 39,999 words
Novelette 7,500 to 17,499 words
Short story under 7,500 words"
In manga, a one-shot would likely fall under "short story" in length, but most series would be at least a "novel" or, more likely, multiple novels.
Comics, not published in Japan, tend to be shorter since it is generally impossible to publish 12-20 pages a week (It's unhealthy for the manga artists too and they even have assistants to help). So for comics one novel would be the usual length for a story.
Hope this helps.