Most people stay at least two years. Historically this was the amount of experience required to get your CPA license. In addition, after two years you are promoted to "senior" where you have the responsibility to supervise first- and second-year staff. Having that experience makes you more valuable in the job market.

(My knowledge of the licensing requirements may be outdated).

Generally people leaving a large firm for a position at a company can expect at least a 20% pay increase, in addition to a better work/life balance. Therefore, the temptation to leave public accounting is very strong.

However the longer you stay, the stronger your resume and the higher the position (and salary) you can command when you leave. If you leave public accounting as a third-year senior, you will probably be able to get a job as a senior financial analyst at a large company, or maybe accounting manager or assistant controller at a small company. If you stay until you are an eight-year senior manager, you may be able to get a job as a controller at a large company, or a CFO at a smaller one.

These are just examples for illustration. It is much harder to move up the ladder at a company than at a public accounting firm (public accounting is "up or out"... if you stick it out and put up with all the crap, you automatically get promoted at specific time intervals). So you are always better off staying in public accounting as long as possible. But that's easier said than done. The lifestyle can be brutal, and if you have a family it may be impossible to have the public accounting lifestyle.

The other thing is eventually you would be considered to be a partner, which is a difficult path because your primary job is to get new business for the firm. A lot of CPAs are not suited to the sales role. There are no non-partner career CPAs at an accounting firm. If you stick it out that long but the firm doesn't consider you partner material, you will be "counseled out" which is a euphemism for being asked to leave.

So... other than the tiny percentage who are suited to being a partner, nearly everyone plans to leave public accounting eventually. It's just a question of when.

But just to be clear... whether you are "counseled out" or decide you can't take the 60-hour weeks anymore, leaving public accounting is hardly a defeat. Having experience at a large firm is extremely valuable no matter which level you leave from. You are likely to have many attractive, stable, well-paying alternatives to consider.