My bonus is calculated by the number of files closed in one month. By 5p on the last day of the month I know how many files have closed and I can calculate my bonus based on that number & know what to expect. How long does an employer have to dispurse the bonus check & can they deduct from the bonus for any reason? If they can deduct do they have to give written reason/explanation and for which files? If a percentage is involved (higher percentage higher $ amount) does the employer have to give written notice to change the percentage amounts? I am currently employed in Ca. Also what can happen if I refuse to sign a write up?

First, a performance bonus as you describe is TOTALLY up to the company. The terms, timing etc of the bonus has to be part of a employee handbook or employment contract. Any changes to this process has to be done in writing.

You do not have to sign a performance evaluation, but generally such a signature simply acknowledges that you received it, not that you agree with it.

There are taxes that must be paid on the bonus so the company will withhold taxes on the bonus check.