From my experience it’s difficult to make good progress on just the weekends. I would say 4 times per week is the minimum.
But don’t let that stop you. If the weekends are all you have, then you should make the best of your situation. It will be a little harder to build momentum and you will likely have to go back and spend more time on earlier steps at the start of the weekend.
What do you do? If it’s possible to squeeze in 15 minutes over the course of your day then I would certainly recommend that. Whether it’s on your way to work, on your lunch break, at the gas station, in the airport terminal, at a bus stop, coffee shop, anywhere…
Tay