OK, try this one: You have a plate of 12 donuts. Every ten minutes, you eat one. How long before you only have six donuts?
TH is is essentially the question before you. Except you're not counting sweets, but detecting them by their ability to rotate the plane of polarized light. Your textbook or class notes should be able to help you with the definition of optical rotation, and what's going to happen as sucrose becomes fructose and glucose, and its up to you to build the correlation.
Looking at it another way. If sucrose was blue in color, and glucose ad fructose were yellow in color, you'd know what the beginning and ending would look like, but what about 10% converted, 50% converted, or the elusive 67% -- they'd all be different shades of green. That's what you'd be looking for.
Unfortunately, we can't see the plane of polarized light. But once we have a machine that can, and it spits out a number, the task is the same.