How often should I practice yoga?

Practice as often as your body can be happy.
Your life is in a constant flux, so the amount of energy your body has to practice is never the same. Forcing yourself to a fixed regimen will burn you out when life itself is draining a lot of your energy.

If you burnout, you quit. Your progress stops there, and you start to deteriorate again. Alternately, if you slow down for a bit but and keep going, you will have the rest of your life to make progress. What you can achieve becomes exponentially greater. The person who walks at a comfortable pace can travel further than the person who sprints, gets hurt and stops.
So it is important that you observe the signals your body provides. When your body resists greatly, rest more. Rest is just as important as practice. Practice provides the signal for your body to change. Rest is when the change actually happens. When you return from your rest, you will actually have become stronger.

​But this does not give you the excuse to be lazy. To get the most out of your practice, you should work on being “truthful” to yourself. When life leaves you with a lot of energy, practice more. When life is draining your energy, practice less. Keep your body happy and you can enjoy a practice that keeps improving for your whole life.