I once derived the equation in the Mark's handbook for sizing a drive shaft from first principles for a project, not knowing I could have just looked it up. I can't even imagine being able to do it now. This is what I miss, 30 years post B.S.M.E. as a programmer. I can't do the math any more, because I can't remember the algebra.
It's been 7 years for me and I'm certain I would fail a test in anything past Calc 1 now. I can still remember derivatives, limits, and basic integrals but anything past that would be a wash without going over a textbook first.
Ironically I actually did really well in multivariable calculus in college. I had tremendous trouble focusing on anything I didn't find interesting, but once I recognized that all the computer graphics papers I was reading at the time were all based in multivariable calculus and differential equations I was hooked.
I share this experience. It seems advanced math skills are hard to "mothball" properly. Either you keep using them day-to-day, or you'll will lose them real fast.
