How much land would the faith own?

It’s hard to say, because the historical context by which the Faith of the Seven came to Westeros is entirely different than the context by which the Catholic Church became hegemonic across Western Europe. A very quick example: there’s no Westerosi equivalent of the Donation of Pepin and thus no equivalent of the Papal States.

Notably the Faith seems to have relatively little political authority even where it has physical structures – the Hightowers rule the land on which the Starry Sept is located, and the Kings of Westeros rule the land on which the Great Sept of Baelor stands, but we can see this even on a more modest level. Despite the fact that Stoney Sept’s economy is probably based around it being a religious center, the septons don’t rule the town – rather, there’s a knight of Stoney Sept. This suggests that the Faith’s landholding hasn’t extended to lordship, which is an important point.

On the other hand, if we look at the septries we encounter in the series, they do have property, both real estate and otherwise: the Quiet Isle has “terraced fields, with fishponds down below and a windmill above…sheep grazing on the hillside,” and has orchards and vineyards besides; the sept where the Brotherhood Without Banners corners Septon Utt was quite large: “Before the war we were four-and-forty, and this was a prosperous place. We had a dozen milk cows and a bull, a hundred beehives, a vineyard and an apple arbor.” And given this is a feudal society, there has to be some sort of formalized relationship that underpins it – but whether that tenure is freehold or something else, we don’t know.

Finally, there is a cryptic comment in WOIAF that “many lords complained of unscrupulous septries and septons making free with the wealth and property of their neighbors and those they preached to,” prior to the Reconciliation of Jaehaerys. So it may well be that the Revolt of the Faithful and the Reconciliation severely curbed the position of the Faith compared to the medieval Catholic Church.