It really depends on how active you are already. I you aren't already active, I would suggest aiming for 2-3 times a week, and eventually bumping that up to 4 times a week. Power walking, hill walks, getting into the gym and doing some weight training or classes (like pump, aerobics, cycle classes, boxing)
And again with the timing of your workouts, the best time is what works for you! If you like to sleep in- then I would train after work or go walking on your lunch break- and if you are a morning person then I would wake up and do it before work. There is no difference what time you do exercise, the biggest factor is that you actually get out and do it!
Personally, I find it easiest to get up and do it first thing in the morning, getting it out of the way and leaving you feeling good and energised for the day. I am always too tired in the afternoon!