Each person will find what suits themselves.
20 minutes, twice a day is plenty sufficient, but if you want to do some more then fine. Be careful not to meditate too much as you can find a) you may get a sense of 'overwhelm' and b) you don't leave yourself time to live your life.
I think quite a few of us here meditate twice a day, using lifeflow for at least one of those sessions, so that's like a 40 minute session and another 20 minute session.
Hugs
Giles