I could hide my drug use for about a year (if shooting) and 4-5 years (if drinking). My last relapse (IV drugs) was Feb 2008 and I went into recovery June 2009. When my dh started "clue-ing" in on the fact that I might be using, I had been off to the races for quite a long while.
Yeah... I found that while I thought I was pretty good at detecting use, my AH was actually much, much better at hiding it than I ever realized. He had years and years of practice on me.