Within a month of starting with recommended routing, I increased day 2 to two sets and day 3 to 3 sets. My idea was to do, at least, as much as I did on day 1. I am happy with that approach. My routine is:
D1 max reps in 20 mins
D2 as recommended by SF but with 2 min rest between rounds and 10 rounds instead of 5
D3 as recommended by SF but 3 rounds with 6 mins of rest between rounds.
When I do extra rounds, I am doing it purely for exercise and they usually take longer than the recommended rounds.