Impossible to say unless you want to do it yourself 😅

The basic Linux kernel patches will be in the next release, and then it depends on when the mobile Linux distros upgrade the kernel.

But I think entusiasm for the FP6 is relatively low in the mobile Linux space, as they removed usb3 and display out from the hardware. I suspect the FP5 will remain the best supported one.

Depends on what you mean by level of support. If you mean hardware, it will never get USB3 or display out. This will make it less attractive as a linux phone, which means it will get less support from the tinkerers.

If you mean community support, after reading a few topics on the Fairphone forums, probably never. To put it in perspective, I know I wouldn't accept a hypothetical free FP5 to FP6 upgrade.