never. Never had a woman ask, never would let a woman do it if she did ask. First, as I always say here, I'd rather she think I'm 8" than know I'm 7". Secondly, most chicks only measure because they want to make the guy feel proud about it, not because they really give a damn about the exact dimensions of it. And I don't really want some broad feeling she has to massage my ego.
I did have a woman ask if I'd ever measured it. I (lied and) said no. She then asked if she could tell me how long she thought it was. I said no. She said "I think you'd like the number I have in mind." I told her to forget it, not interested in her estimate.
Just gross, IMHO. How many women would be ok with a dude whipping out a measuring tape and measuring her bust, her waist, her hips? Not many. And how about shoving a ruler in her snatch and measuring how deep she is? Can't think of any woman who'd be ok with that.
To me it's none of my friends'/lovers' business what the exact dimensions of my body parts are.