I've only been playing for a year, so I'm not sure what my advice is worth, but I figure as long as you leave your team in decent shape (not maxed out in payroll and/or devoid of talent), you're okay to walk away.
I'm in a world that isn't very good or very competitive, and it took almost two months to fill last time. Worse, one of the owners in my division is winning 110+ games per season now because he tanked for 5 seasons to collect superstars. But I'm not leaving it until I clean up the mess I made. It was my first team, and in my first season I made some stupid free agent deals that won't expire until after next season. I figure I owe it to the other owner to leave a team that isn't a complete trainwreck. I agree with Mike that its kind of a jerk move to leave a mess for the next owner.