I bought a pair of Naked & Famous lightweight (10oz) selvedge jeans to wear at work.
I'm working in a trailer at a mine site for the next 3-4 months, so I figured I could wear these all day and break em in.
I'm out walking around the site a lot more than I expected, so they're getting mud/snow/dirt on them, should I just let em get real gritty and wash em at the end?
Not sure if I read your question correctly, but I think you're asking, "Should I wear these pants, day in and day out, for 3-4 months, let them get very dirty and wash them at the end?"
My answer: absolutely not. The thought of wearing the same [fill in the blank for any item of clothing] for 3 or 4 months is ridiculous. Wash the pants every week, whether they fade a bit or not. For both hygiene and aesthetics, wear clean clothes (you'll also have a better chance of getting the stains to wash out if you don't allow them to "age" until next February).
And while we're at it, you're not homeless or destitute (I assume when you said "at work," that this is a paying job). Naked and Famous jeans aren't cheap. So go buy yourself a couple of pairs of Levi or Lee jeans, rotate them, keep them washed and you'll be fine.