Once a day is about right. It helps to make it part of your routine — so if you find it easiest to meditate in the morning before breakfast, try to stick to that (and it’s completely fine if that means sitting on a Saturday about 3 hours later than you do on a weekday). Needless to say, if you’re feeling enthusiastic you are very welcome to do more, but if that’s the case, try to do several shorter sessions rather than attempting one long marathon session. Do remember that you can take your meditation anywhere with the Headspace app so this might make squeezing it into your busy day a lot easier!
For more help on this question and many others, please check out our videos from Andy on our YouTube page here.