I would follow the meditation sessions in this program. The first session is a five minute intro; the second is 10 minutes. Some schools of mindfulness meditation believe you never have to sit longer than 20 mins a day.
In general, I would say, start with 10 – 12 mins a day, then increase by 2-5 mins a week, no more.
And always treat yourself with loving kindness (metta)
I would say maybe 10 minutes is probably a good amount, but I don’t think there’s any one specific answer.
So not committing to a long period of time usually will make it easier. As you learn to let those thoughts drift back, then you can increase your time. Meditation is time you are taking for you during your day. You should feel better after, even if you can't pinpoint why. I find 5-10 minutes tends to be best for me. It helps me reset and get energized for the day. Much longer and it feels like work. Maybe in the future, though, fifteen or twenty minutes or even longer may be beneficial for me.
Let it take its own time in become an "effective" part of your life. If you feel restless, jumpy muscles while doing it for example, stop. And avoid becoming "addicted" to it, to the exclusion of that which must be done, unless you have the time to ferret out the meaning of life in a monestary or complete to Unified Field Theory at a Hadron Collier.