Two ways to measure it
How is the rate of a reaction actually determined? The formula holds two quantities — an amount and a time. So there are two main methods: fix one and measure the other.
Method i — fix the time
Measure the amount of reactant used up (its mass or volume), or the amount of product formed, within a fixed period of time.
Example: measuring the volume of carbon dioxide collected in 2 minutes.
Method ii — fix the amount
Measure the time taken for a fixed amount of reactant to be used up, or for a fixed amount of product to be formed.
Example: timing with a stopwatch how long the piece of limestone takes to disappear completely.
Which of the two to choose?
It depends on what is easy to measure. Method ii is convenient when the moment the reaction finishes is obvious; method i is convenient when the reaction simply keeps going.