I usually use buttermilk beyond the expiration date. I am wondering what other people do and how much time they let pass until they ultimately...

It depends on what I'm using it for, how far beyond the expiration date it is, and whether it's been kept in the very back of the fridge, where it's coldest. I go through a lot of it, because I use a quart every ten days or so for making bread, salad dressings, muffins and other baked goods, etc. I have come back from vacation, however, to find a half quart in the back of the fridge, a week or so past the date on the carton, and it's been just fine for baking. Look at it and smell it. If it looks okay, though a little thicker, and smells okay, though a bit more tangy, it should be fine. Shake well, though, before using! ;o)