Is it Illegal Not to be Paid Hourly?

I recently accepted a job as a housekeeper for a small local hotel. I was told I will be paid minimum wage ($7.50) for every 2 rooms cleaned. So ideally it should take me 30 minutes to clean each room. However, since I'm new to this job it takes me at a minimum of 45 minutes to clean each room. My husband believes I should be paid for every hour I work, regardless of how long it takes me to clean each room. The way they choose to pay their housekeepers may be illegal. What are your thoughts or legal opinion of my particular situation.