Where to start: a rhythm for the first month
The first month is about getting used to it, so there is no hurry. One or two classes a week give the body enough signals to start changing and leave time to recover. If the first class went well, add a second a few days later.
Space the classes out so there is a rest day between them. Muscles that worked in unfamiliar poses can catch up, and the habit settles in without overload.
Make a simple timetable, say Tuesday and Saturday. Fixed days are easier to keep than "whenever", and easier to negotiate with family and work. After a month, see how you feel and decide whether anything should change.
How long each session should be
A standard class runs from an hour to ninety minutes, and an hour is a fine amount for a newcomer. Short twenty-minute sessions work too, especially at home, because they keep the thread going between classes.
There is no need to chase long sessions. If you finish feeling you could do a little more, the amount is right. If you feel dizzy, stop and rest, and see a doctor if it keeps happening. If your joints ache, shorten the session and simplify the poses.
Part of that hour goes on the warm-up and savasana, and neither is wasted time: they make the main part safer and more pleasant. If you are really short of time, trim the main part rather than skipping the rest.
If you have a desk job, short breaks through the day count too: stretch, open your chest and take a few slow breaths.
A tip: twice a week beats one rare marathon
Two sessions a week of about sixty minutes, plus a few minutes at home (one or two are enough in the first month), usually serve you better than a single marathon class once in a while. This is a general rule of thumb rather than a law, and everyone's schedule and energy differ.
The reasoning is simple. The body learns through repetition, and a rare large dose is more likely to bring soreness than benefit. Small but frequent practice builds strength and flexibility, and the habit holds more easily.
The home minutes can be very simple: a stretch in the morning, a few rounds of cat-cow or a rest in child's pose. They do not replace a class, but they keep you connected to the practice on days when leaving the house is too much.
Building up and combining with other training
Once two classes a week feel easy, add a third or make the poses a bit harder, for instance by holding them longer. Change one thing at a time: frequency, length or intensity.
Yoga pairs well with running, strength training and swimming, since it loosens muscles and develops balance. After a hard workout, choose a gentle class such as yin or restorative. With an injury or pregnancy, talk the frequency over with your doctor and teacher.
It helps to jot down a few notes: how you slept, how your muscles feel, what your mood is. Those notes tell you more about your rhythm than any general advice.
On strength-training days, for example, you might do a short stretch for legs and back, and on rest days go to a full class. That spreads the load more evenly.
When to take a rest day
Tiredness is information. If your muscles ache more than usual, your sleep slips or you simply dread going, take a break or pick a quiet practice. Rest does not take results away; it helps them settle.
Even on non-class days you can stretch for ten minutes or lie with your legs on a chair. That keeps you in touch with your body without adding load.
If a class gets lost to a busy week, do not try to make up for it with a double dose next time. Just return to your usual rhythm.