An hour or two. I have magic sweat, and, apparently, alot of it. If I shove a blue gun in my waistband, in the summer, it will have surface rust in two hours or less, no matter how oiled it is.
All my stainless guns have rust under the grips even though I take the grips off, brush all the rust off and coat everything with Boeshield once a month or so.
Stainless slide? Maybe a week. If I carry my P239 in my waistband, in a holster, I will find rust inside the barrel in a couple days.
Its aggravating as hell. I rust through hard chrome.
I just assume I have a very efficient system that purges toxins out of every pore in my body.