As detailed as they want. My friends and I go on about this stuff all the time. But it depends on how close we are as friends. We'd have to be close enough that I talk to you about my relationship in general...the good stuff as well as the not so good stuff.
I wouldn't tell some random acquaintance how good my boyfriend is in bed just like I wouldn't tell them when we're going through tough times...for obvious reasons. Not that I'm insecure, but some people are opportunists.