If you just want the occasional stretch, then once a week is fine.
If you want to lose weight, get fit, develop flexibility, improve your energy, sleep better, improve your mental and emotional health then ideally practice 3 times per week or more.
The more often you practice, the sooner you’ll get the results you’re seeking. Also, the calming and relaxing effect of yoga will wear off pretty quickly if you don’t practice several times a week.
If you’re seeking a change, you have to change your routines and that might include finding some more time for yourself to practice at home or in the studio.
It takes more than a few yoga classes here and there to make a difference to our physical and mental health.