I used to advise friends to stick to the 25 / 5 minute break, but more because I thought that this enabled one to sustain this rhythm for longer. However, currently I'm experimenting with longer pomodori, with the reasoning that if I'm in the flow / deeply working, the value of that flow is too great to take the risk that I might break it because of an arbitrary break.

I think a better advice now would be to experiment, and see what suits you best in terms of sustainability, output and fulfilment.

Today I my pomodori were between 25 minutes and an hour. It's home time now, but I feel great due to a large part of the day filled with flow.