Well, that's not something that can be predicted precisely. It could continue for a while if the story has more to unfold or come to an end sooner if the plot reaches a natural conclusion.
The Chainsaw Man manga has a considerable length. It consists of multiple volumes and chapters, but the exact number can vary depending on how you count and which editions you refer to.
I don't have the exact figure off the top of my head. But a quick online search or checking with manga enthusiasts should give you the answer about the number of Chainsaw Man manga volumes.
To make your OC stand out in Chainsaw Man fanfiction, focus on their backstory. Maybe they have a tragic past that ties into the world of devils in a different way than the main characters. For instance, their family was sacrificed to a devil not for power, but for a strange religious cult. This would shape their motives and actions in a unique way. Additionally, their power should be something that hasn't been seen before. It doesn't have to be extremely overpowered, but it should have an interesting mechanic, like being able to swap places with a devil at will.
I don't have the exact number of chapters for Chainsaw Man manga off the top of my head. Usually, you can find this information on manga-related websites or by referring to the publisher's updates.