How often are Quidditch games won because of the Snitch being caught?

Presumably, almost every game of Quidditch ends because the Snitch is caught. As Oliver Wood explains in Chapter Ten of Philosopher's Stone:

A game of Quidditch only ends when the Snitch is caught, so it can go on for ages – I think the record is three months, they had to keep bringing on substitutes so the players could get some sleep. "Well, that's it – any questions?"

This is confirmed in Chapter Six of Quidditch Through the Ages with one addition:

A game of Quidditch only ends when the Golden Snitch has been caught, or by mutual consent of the two team Captains.

There is apparently at least one example of this happening, mentioned in the same chapter in Quidditch Through the Ages:

There is a tale that a Golden Snitch evaded capture for six months on Bodmin Moor in 1884, both teams finally giving up in disgust at their Seekers' poor performances.

It would seem that it is also possible to end a game by forfeiting the match. In Chapter Ten of Chamber of Secrets Harry is concerned about this:

"If we stop now we'll have to forfeit the match!" said Harry.

And according to Chapter Six of Quidditch Through the Ages a team is disqualified if it fails to return to the pitch after a two hour timeout:

Time out may be extended to a two-hour period if a game has lasted more than twelve hours. Failure to return to the pitch after two hours leads to the team's disqualification.

But, for the most part, we only ever see games that end via the Snitch being caught.

In terms of the Snitch being the cause of victory as opposed to merely the cause of the game ending, the statistics probably don't change much. There are at least two examples where the team that caught the Snitch lost the match. In Chapter Eight of Goblet of Fire Bulgaria catches the Snitch but Ireland wins:

"IRELAND WINS!" Bagman shouted, who like the Irish, seemed to be taken aback by the sudden end of the match. "KRUM GETS THE SNITCH – BUT IRELAND WINS – good lord, I don't think any of us were expecting that!"

And in Chapter Twenty-Six of Order of the Phoenix Gryffindor caught the Snitch but Hufflepuff won the match:

The miracle was that Gryffindor only lost by ten points: Ginny managed to snatch the Snitch from right under Hufflepuff Seeker Summerby’s nose, so that the final score was two hundred and forty versus two hundred and thirty.

But this seems to be a pretty rare occurrence, as evidenced by the surprise at the Ireland vs. Bulgaria match:

"They'll be talking about this one for years," he said hoarsely, "a really unexpected twist, that....