How long before I can leave my JRT and Kitten alone?

I recently got a kitten, she is almost a year old. When I first introduced her to my 3 year old Jack Russell, she would hiss at him and he would wine. The second time, they seemed to be fine together. It's been a month since we've had her, but I'm not sure how much longer I should wait before leaving them together while we're not home. They seem to be getting along pretty good, I can tell that my Jack Russell is a bit uncomfortable around her, especially when she tries to rub up against him (he growls a bit too). She's very play-full so sometimes she will jump on him and bite him, he only growls a bit. He's never tried to snap at her. We used to have guinea pigs, and he tried to snap at them no matter how much we disciplined him (Probably because JRTs are hunters) He used to growl at stray cats outside but now he seems to be more interested in birds.