Lucy won't even go outside by herself to pee so she's never outside alone...there are times where she just wants to lay in the sun, but it's been so hot and humid here lately that I get uncomfortable just sitting there waiting for her to go potty, so as soon as she's done, we come inside.
We let Molly out whenever she wants out. Usually she goes potty and then wants to come back in. But in the summer when we have the a/c on, she gets cold so she'll go outside and lay on the deck, and depending on how hot it is she's sometimes out there for close to an hour. When she gets hot, she comes back in.
I would never let them outside and leave though. If they want to go outside, I'll let them out and go back to whatever it is I was doing. But when I see that they want in, I let them in, whether they've been out there for 5 minutes or 30 minutes.