I no longer work for Nokia and I'm not sure how this is even a topical response, but I'll go ahead and answer anyway: never.
Nokia phones are still proprietary hardware and even if they were to be able to run an open source symbian version (there isn't one yet), the adaptation layer is still not open source. Ignoring that, S60 itself isn't open source. Ignoring that, Nokia has always attempted to make custom firmware exceedingly difficult (storing flash images as partially encrypted to a specific asic serial number for instance).. of course that may no longer be true. If the encryption is no longer required in the Nokia hardware, then I suppose custom images might be possible by altering binary components in the image. But I hardly think you should 'expect' it to happen.