I don't think there's a certain amount of time, it just depends on how well things are going and if you're comfortable with having the person seeing other people or not.
With everyone I know, if you call it dating that means you are boyfriend/girlfriend and are exclusive. Before that, you're seeing each other. But if you're seeing someone it may or may not be exclusive.
My boyfriend said he didn't want to see other people after 3 dates. I wouldn't have been seeing other people while I was seeing him anyways though. I don't think he had actually been seeing other girls since our first date either Then I found it kind of confusing because I wasn't sure if that meant he wanted us to be boyfriend/girlfriend or not. But apparently it didn't mean that, because later he asked me to be his girlfriend.
So I think it's important for people to really spell out what they want to avoid problems. Like you said, some people would consider it cheating while other people would see no problem with it, so you have to make sure you're on the same page.