Not sure, but I assume a couple of years at least - it might be also affected by stupid external factors, like insane tariffs and such. Also it will take some time for DDR6 perf to go up - for both mobos and memory. So even at the start you might be still better off DDR5 and if you go with DDR6 you might need to replace mobo and memory to get better perf. That's my impression on hardware situation, I might be wrong, though.
What is your use case for threadripper, I'm curious? AFAIK it's not a good match for gaming at least. Or is it?
How time flies, eh? I went searching for more info - https://www.techpowerup.com/339178/ddr6-memory-arrives-in-2027-with-8-800-17-600-mt-s-speeds It also mentions an architectural change to increase the speed further.