However many miles you cycle normally - I regularly do 65 + miles during the summer out on the road bike - you won't need to spend that long on the turbo for a good workout. The most I've ever done is around the 1 hour mark.
A good start is to ask yourself what you want it for..? This sounds obvious but I can tell you now that if you think you'll be on it for hours on end, you won't be. It's no replacement for a ride out on the road proper but it's good if you want intensive, focused training sessions, it's too foul to ride or if you are limited by time.
My first turbo was a magnetic one and I didn't like it at all but I had no purpose, I just thought it was a good substitute for a ride out and so I soon got bored given the lack of structure. Now, on the other hand, I have specific training goals and less time than I did (and I have a fluid turbo), the whole thing makes a lot more sense.
Have a think about how you want to use it because as an end itself it's pretty boring to be honest. On the other hand, you can get a very good workout in half-an-hour if you have a focus.
Good luck.
(And I've just seen Black 'n Yellow's post which says roughly the same thing!)