I think 45-60 minutes is just fine. Before I had kids and a husband, I worked out at least 90 minutes a day. It's different for me now, though. I have to workout before my kids get up, and I don't have enough time before work to workout for much more than 60 minutes -- and that's with getting up at 4:15! And it never seems possible for me to get in any additional workout time in the evening after work. Between fixing dinner, my 2 young girls, getting ready for work the next day, and getting to bed at a decent hour (so I can get up early), my evenings are toast. I totally OK with 60 minutes (or less), though, since that's what fits into my life.