How long can employer withhold pay?

I work as a freelance location Scout and Manager on TV commercials. My work sometimes begins weeks before the actual filming occurs. Most of the time, my clients use a payroll company, (to have an employer of record,) to pay me. They typically hold my time cards and they don't submit them until the end of the shoot. If I invoice the client, I understand that I have to wait for payment at their convenience. If I'm filling out a timecard, I'm, suspecting that they should be submitting them weekly to the payroll company. I'm assuming this, because numerous times my work dates have been falsified on time cards that the clients complete and then have me sign. If I'm signing a time card, is it legal to make me sign falsified work dates and have to wait weeks before the time cards are even submitted to the payroll company?