It's different for every person. It sounds like you have an "open relationship", and if that works for you and her, then great. If it doesn't work for you, then it's better to tell her in case things get out of hand later.
If you're worried about walking out on her and then having regrets, well, that's life. Either you stay with a girl and put up with the parts of her you don't like, or you dump her and never look back, and if you have regrets, you just have to deal with them. In my case, the cons of staying with my girlfriend outweighed the pros so much that I'm happy I left her, even though it meant I was alone again. I'm fine being alone. But some people aren't. You have to judge for yourself.