There's no minimum or maximum time frame for telling someone that. All the women's magazines and Sex and the City and things like that try to tell you there are 'rules' for all that shit, but there's not. It's all up to the individual.
I would suggest for you to wait just a little bit, only because you said you 'think' you're in love with her. Perhaps wait until you can say to yourself 'I fucking know I'm in love with her', because you may actually feel more confident saying it when you know for sure.
Good Luck :)