I used to think a 20 minute workout was all I needed. And granted it's better than not working out at all. There's been plenty of times that all I have is 20 minutes and I use them wisely, but overall and normally I work out 45 minutes - hour 5 days a week.
I usually get to the gym by 4:30 p.m. and go straight to the treadmill. I run at least 30 minutes and get around 3 miles in, (sometimes a little more), then hit the weights for another 15 - 30 minutes depending on time.
I have this schedule set so I can get Bella from school before, let her play with her friends at the gym while I work out, then get us home, bathed, & dinner on the table by the time The Hubs gets home from work. (Trust me there's many times dinner's not ready when he gets home or I call him and tell him I'm exhausted and he picks something up for us on his way home!)
Obvi, the more time you use to work out the more of a workout you get, but at least try 30 minutes a day. I saw this on Pinterest today & although I'm not sure if it's completely true, it's definitely something to think about when working out...