Where I work they don't allow any, at all. So if you're out, say to take care of a sick relative for a few weeks, you can't make up any time, or get credit for trying to stay up to date. Even if there's something urgent and you end up spending hours in teleconference and then more time working on the issue, you're not allowed to put those hours in to your time sheet as time worked.
When I was there as a software engineer, I worked from home one day a week.
But at least at a few years ago, you couldn't work from home full-time forever. I knew folks in the Atlanta office, and after it closed they could work from home for a long while (many weeks), but needed to eventually find a position at another office or leave the company.
It comes down to how lenient your manager is, but the official policy was you're only allowed to work from home in exceptional circumstances (like you have someone coming over to work on your house), and they look askance at doing it more than a couple times per month.
Ironically, the Atlanta office is now expanding again. Turns out that closing it was a big mistake re: making diversity goals. I could've told them that ...