I'll put on my personal trainer hat to answer this one: it is really an individual thing.

I personally train 6 days per week - for about 1-2 hours (i have a 'fun' job as a fitness leader in classes like body attack, body combat, body balance, body pump and RPM - oh, and body jam)....depending on the hours i work at my 'not fun' job etc - and how many classes I get at fitness centres.
Most reports I've seen point to toning down the exercise - but still keeping active. High impact exercise is great fun - but it can put the body under great stress too. Most recommend light impact exercise eg, 30 - 45 min walk session, light exercise bike riding, pilates or a fitball class, body balance is a good class to take.

I have toned it down to a total between 4 - 6 hours total per week to play it safe. I make sure I have 'rest' days inbetween training sessions too.