I relapsed after 4 years clean/sober, and not only did I pick up where I left off, but in a short time was as bad as if I had drank those 4 years. It was horrible and a lot of those 'yets' I hadn't experienced the first time I got sober came to fruition, such as blacking out and driving myself home. My mental state in between binges was unbearable.
I've also known people with 20+ years who have gone back out, and died within a very short period of time.