The answer to the question is It depends. (Don't you just love those definitive answers?) For years and years, the rule was that a resume had to be no longer than one page. That made sense back in the days of the typewriter and hardcopy resumes that had to be kept in storage for at least six months. The less paper the better! The one page rule was also easier to keep when most people had a career track that included only one or two positions that were long-term.
In this day and age, people change jobs quite often. It's not unusual for someone to change jobs once a year, even. The national average is somewhere around every 18 months. Part of that flux is due to a changing economy, but some is a result of the general quest for something better - better money, better benefits, better life balance, and better challenges. Therefore, within a ten year span, a person may well have held five or six jobs, each one carrying its own details and accomplishments. Getting all that into one page and have it be an effective resume is impossible.
But what if you have twenty-five years experience? Should you take up four or five pages giving the details of everything you've done back to the age of REO Speed wagon and big hair? No. Employers are primarily interested in the most recent ten years of your career because that is what is relevant.
Relevance is a concept with which many job seekers have a problem when trying to construct their own resume. They can't see when information is relevant or not so they end up including everything they can think of in the resume. Resumes end up being pages and pages long because the job seeker has included everything from his Eagle Scout honor to the fact that he made coffee for the staff meetings on Friday mornings. Objectivity toward relevance of information is a benefit that a professional resume writer brings to the job search.
I still haven't answered the original question How long is too long? It's too long when you have so much information that the reader cannot grasp the main points of value of the job seeker in the 45 seconds he/she spends on the resume. A general rule of thumb is the average resume is two pages in length. That's not a rule, but an average. Executives with extensive experience and earning a high salary may well have a three page resume. If you find your resume runs beyond three pages, you should go back and look at relevance of information. Does every bit of information that you have included in the resume have a direct purpose in winning an interview for your current job target. If not, take it out.