'It's been a while' is standard English. It's the first part of a sentence such as 'It's been a while [since we last met]' or 'It's been a while [since we last spent time together]'. You might use it like this: 'Hi! I haven't seen you for ages.' 'Yes, it's been a while. How are you?' 'Long time no see' is not standard English. In fact, it's a collection of unrelated words, based on Chinese 'pidgin English'. Grammatically, it makes no sense. Everyone knows it because it's such an odd expression, but it in fact not many people use it - at least, not seriously. When people do use it, it's a kind of jokey greeting. 'Hello, stranger, long time no see!'. You would never say this in a formal situation.