My employer pays bi-monthly. Does that get him out of paying overtime?

I think you mean semi-monthly (twice per month). No, that doesn't do it. How many employees in this company and what type of company is it? I'm trying to determine if the employer is even subject to the FLSA.

There is 7 now. It is a dentist office. Ane he pays on the 15th and 30th. I was recently told that I should take the number of days in a pay period and multiply by 8. ANd anything over that should be considered overtime.

It's likely that your employer would be subject to the FLSA, as one of the criteria that, if met, makes the employer subject, is that the employer have interstate commerce dealings. Such dealings would include ordering supplies from out-of-state vendors, accepting credit cards (which are likely to be issued by out-of-state banks), etc.

As such, the employer must pay overtime to nonexempt employees if they work over 40 hours in a workweek. The employer must define the workweek as 7 consecutive days, beginning and ending at a time certain. The frequency at which you are paid is irrelevant. And I don't know where the multiplying by 8 came from; it's nothing in the law.

Are you a nonexempt employee? What do you do there?

I am the dental assistant. And he does have set hours every day, I don't believe he ever defined the workweek as 7 days. Is there anything I should or is it a lost cause?

As a dental assistant, you would be a nonexempt employee. You can file a claim for unpaid overtime with the state Dept. of Labor.

If the employer has not designated the "workweek" (which he is required to do so that overtime can be determined), it's likely the DOL will set one for him.