Once or twice a day is fine.
As with all meditation practices, it's best not to overdo it. Some people think that the more they meditate the easier or quicker they will reach 'enlightenment' (whatever that is to them). Not so. If you spend all your time meditating, you're not spending time living.
Find the right balance for yourself.
For me, Lifeflow for 40 minutes in the morning and a 20 minute meditation in the evening is plenty sufficient.
Hugs
Giles