Can someone help me understand the power of Thor's potential.

I know King Thor will have more raw power than Odin since he inherits his father power in addition to his own and over time, he will learn to utilise his power but, what exactly is the upper limit if this considering he is also half Elder god?

Umm, i'm pretty that if he will unleash his full power he probably just would destroy half of the universe... accidentally. (without Mjolnir)

His upper limit is up to whichever writer is handling him at the time. But at the very least he'd one one bad-ass Skyfather once he inherits the status of King from Odin (which probably wont happen in comic time for a few thousand years).

Well, there was that time he became Rune King Thor and pretty much restarted the universe... I think. I know he was pretty up there.

when and how is it decided when odin steps down, and why would he ever need to if they are immortal?

he has more potential than his father due to his elder god lineage