Depending on your situation, you can use that phrase in multiple ways. One example is to use this when you feel disappointed about something and you want to leave you can just stand up and say "I'll leave now". "I have to go." is more of a polite way of making the statement that you want to leave. But I would suggest using the full version "I will leave now." or even "I'm going to leave now." However, you could just stand up and leave without saying a word and make the people think about it.