Two 20-minute sessions per day are wonderful: once shortly after waking up when the mind is still clear, and another at the end of the day when the mind is winding down. But this won't be a perfect fit for everyone.
Be realistic with your meditation practice and do set a schedule for yourself, no if's and's or but's. Skipping one day of meditation is hazardous, so take it seriously. If you're on a 2/day schedule, and one session falls away, that's not quite as disastrous as no session for that waking cycle.