For me, how long I can go (how many intervals I can do) often varies from day to day. So I don't try to have any died in the wool, must-do time limits. I aim for a minimum of 5 intervals, and I thinkI've only ever once not been able to do 5. Usually I do 6 or 7, which is a little over 30 mins for me.
But again, it just depends. If I'm tired, or I didn't eat right the day before (or the day of), or if I've been travelling (I often travel east coast to west or vice versa, so I get jet lagged), or whatever, it affects my endurance.
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