It is appropriate when you feel it. But it might not yet be the right time and place. In general I would say you can tell it when should have already an established basis of trust and attachment. But you cannot know basically as there is no telepathic mind reading. So it always comes with a risc. Be prepared to confront the other person with this when they are not yet ready for this. In general I assume it typically takes some weeks or months until you can assume your emotional basis is ready for this. But it also depends strongly upon your personality. Also there are attachments that are based on mutual trust, liking and understanding without big love. This type of relationship is not a minor thing. It is also worth being kept and cared for. Also there is a love that just exists based on physical attractions. This is not just the friends with benefits-thing. You should know what type of love you got. It is up to you to talk with your friend about it. More than half of a working good relationship is good communication with your partner or maybe future partner as well.