I don't recall a specific temperature, but my (often faulty) memory says the regen limit line would appear anywhere around 10C. The last four winters the regen limit has consistently showed, and I have the same weather (well... less rain and more flowers ) as you.
I just wish they added a deadband for the regen warning. When it is almost at temp it keeps going away then flashing the warning constantly for at least 10 min of drive time