It purports to be an Xmas sale, so I’d guess from now until maybe Tuesday? Perhaps to the New Year, but then it might more logically be called a “holiday” sale.

Frankly, this being a community support site, probably any feedback you get will be supposition. Your best bet would be to call in to their sales folks directly, who would likely have a more concrete answer for you. 

In the UK, the current 20% offer ends at the end of today (23rd Dec)

This was documented on Sonos emails, in addition to sponsored Ads on Facebook/Instagram etc.