Almost never. Exceptions are when they need to make the roll repeatedly, say, for a long difficult climb up a glass mountain under an antimagic field, or for tightrope walking across the grand canyon (things that have been known to come up in my games). Then I'll just tell them, "DC is XX, fail by Y or less and you make no progress, fail by more than Y and you fall." or some such. Speeds things up.
Otherwise, no. They don't get to know. If they figure it out, say because they have to keep making it, and sometimes fail and sometimes succeed (and have found the exact threshold), I don't mind them knowing, but a table rule is that you don't say it outloud. Breaks the mood.