I just add it in the morning when I feed my fish. Supposedly Excel will last for ~24 hours in the tank, so as long as you add it at around the same time each day you should be fine. Excel isn't CO2, so timing isn't as important.
It can accumulate over time so I'll skip the dosage every 4th day.
I don't use recommended amounts either - I dose 1 drop per gallon for the small tanks and 1 drop per two gallons for the larger 20+ gallon tanks.
According to Seachem, Excel is broken down by organics in the tank, as well as being taken up by the plants. So the higher the bioload and the more plants you have, the lower the Excel in the system after a set amount of time. If you're doing regular water changes, it is unlikely that it'll ever have the chance to accumulate to dangerous levels if dosed at the recommended amount, let alone the 1/2 to 1/4 that you do.