Read your state law on the subject. It varies by state. Some states allow an additional small charge per day up to a statutory limit.
Having said that, it is not a wise business practice to keep allowing rent to go unpaid and tack on additional fees. After your 5-day grace period comes and goes, you impose a flat fee AND issue a 3-day pay or quit notice. That way, if you do not have the full rent plus the late fee by the 10th, you may begin the eviction process for non-payment. Giving someone more time for an additional fee doesn't generate more revenue, it just drags out the eviction process.
Most states would not let you keep piling on until the 20th, and why would you? If they don't pay by the 20th, you STILL must issue a 3-day pay or quit (on the 21st) which means you can't begin the eviction until the 25th. You've lost half a month.