From what I can gather a comet like Halley's could be visible to the naked eye for around month as it approaches the sun (
Halley's Comet - Wikipedia). Now the physics of an object many times larger than Jupiter would be quite different and we would probably be seeing it partly because of the sun's light reflecting off of it and partly because of electromagnetic phenomenon, so I expect it would be visible for much longer, but if it has an elliptical orbit like a comet it would speed up as it makes its closest approach so we might expect a period of 1-2 months of maximum visibility preceded and followed by a longer period (months? years?) of lesser visibility.
So if it does "appear" in the short term (ie in the coming weeks or months) it would make sense to me that it would take months for it to traverse the solar system and disappear again.
I'm just speculating based on the little I know about astronomy.