I think its partly up to the cats to decide, I will keep them as indoor cats for as long as they are happy with it, which could hopefully be their whole lives. our house is a good size with hallways and 2 sets of stairs so they can run about and the spare room will be set up as a cat playroom with a cat tree, their own comfy armchair and my computer desk and chair. If they seem unhappy living inside then I would be prepared to introduce them to the outside, but only when I am home and preferably when I can be outside with them. When I lived with my parents, we once lost Casper and he was gone for 4 days, he had a cat flap and could come and go as he pleased, but one day he didn't come home. I was in bits the whole time, I'd come home hoping for him to be there and cried when he wasn't. My birthday was during this time and I didn't enjoy it one bit, however the day after when I came home in the evening, there he was sat in the middle of the kitchen table, very hungry but no worse for wear. We decided that he must have got shut into someones garage as he was always wandering into ours. It taught me how much it hurts to not know where your kitty is and that they could be hurt somewhere.
The worst part was that the day after we got him back, a lady from down the road called to see if we had found him. We had put out flyers asking if anyone had seen him and she thought she should warn us that she lost her cat a few weeks before and he was found dead a few days later after eating rat poison. This upset me a great deal and I felt so sorry for the lady. There are just so many danger outside!