The two-minute rule has its roots in GTD: If you can do it in less than two minutes, do it now (assuming you have no other, bigger priorities at the moment.) Over at the Buffer blog, James Clear adds another rule: When you start a new habit, make your goals into two-minute bites so they're easy to do any time.
No time to write? Not even TWO minutes?
Everyone's got five minutes a day to write. Start small. Start with two minutes, and build your writing habit.