IMO, relapse IS a very subjective concept. It depends on the person, the situation, and the substance involved.
I occasionally hear stories of alcoholics with over a year of sobriety who had a beer with coworkers after work and it didn't trigger a full-blown fall into old patterns.
Or a recovering crack addict who smokes a joint on an isolated occasion and doesn't feel the urge to buy an eight ball.
They are in the minority, of course, as most of us would go right back to our bad habits when placed in similar situations. I know I would. So why risk it?