I've relapsed twice after long-term sobriety (5+ years), and looking back, both of my relapses were brewing for months. I had stopped being proactive in treating my alcoholism and doing the things that had kept me sober up until that point. Life had gotten infinitely better and I took my sobriety for granted. Both times a "life crisis" came along that I wasn't prepared to deal with, and I reverted to dealing with them the way I had in the past, by drinking.
The first time I relapsed it took a year before I got sober again. The second time I relapsed it took EIGHT years before I got sober again and I nearly died. I have to remember that I will always be an alcoholic, but there are some simple things I can do daily to live a relatively happy, sober life free from alcohol and drugs.