According to Joint-CEO of CDPOR Michał Nowakowski:

Three months after the release of Hearts of Stone & Blood and Wine we were at 22% and 24% att rate. We landed between these values with Phantom Liberty in a similar timeframe

TBH, the decision to go through with the expansion so long after launch of the single player game was a bit counterintuitive. But what happened is a proof to us that solid content that people are happy with is the way to go.