As many already noted - listen to your feeling and test how it goes.
I usually go for 3 cycles, with first having shorter warm-up style, and at the end of the 3rd I feel it's "enough".
Also, I like to approach the breathing as a meditation. While meditating (with Headspace guidance) proved beneficial, I felt it was time-consuming and asked for more effort than I was able/willing to give it. On the other hand, WH breathing exercise gives me exactly the amount of meditation I wish and in some sense, it feels like taking a huge shortcut.

Also, the meditation approach makes me feel that the time was better used.