If I grab some ERC20 Elrond, is there a definitive date that I need to convert it to native Elrond?

Better hold native egld instead

The bridge from ERC20 to native eGLD was meant to be closed since last November. It hasn't been closed and it can still be used at the moment but it is unknown for how long. Native Elrond offers more options though, like staking and farming, so there really is no incentive to use the ERC20 version.

Questions in my mind is who would even be selling that? How would there be a market for it? If there's much of that for sale, it has a good chance of being bogus ERC20 EGLD, and thus not convertible.

Make sure it's ERC20 EGLD and not ERD. ERD can't be converted anymore. If at all you buying.