Possibly never. Sonos don't make speakers they make multiroom audio systems controlled by Sonos software. Sonos have given no indication that they intend to make integration with other systems possible.  That doesn't mean that they won't,  but I wouldn't hold your breath. 

Thanks John. That’s not good

Well, it could happen if one company purchased the other, but generally speaking, disparate companies don’t cooperate with each other that much. They’re more interested in selling you their own hardware, and not letting you purchase from some other company. 

Outside of one company purchasing the other, perhaps if Sonos gets a large enough license fee for using their multiroom audio IP for each echo sold, then perhaps it would make sense for Sonos to be on board with this.  But I can’t imagine amazon wants to lose any profit on their echo sales, if they make a profit at all, unless they really have to.