Impossible to answer unless one is intimately acquainted with this particular couples eating habits.

How much do they eat per day?
Multiply that by 365.

Agree impossible to predict. We take our coffee and tea black, and have waffles or corncakes/pancakes just a few times a year. But use our maple to replace sugar in everything else...but not heavy sugar lovers. Rhubarb BBQ sauce, dressings, a bit as a pie sweetener a few times a summer, etc.
We made just over a gallon this past spring and may have made 2 gal if the weather had been better. 8 taps over two weekends was all we had time for. One gal is enough for the two of use but hope for more next year to share with family and neighbors.