Really in love

The number one rule for anyone for when to say those three special words "I love you" would be for them to wait to say it until they are sure they really feel love for the other person. Many people confuse love with infatuation or strong liking. While some relationships may survive a person saying a premature "I love you" because the strong liking develops into love later, other relationships may never make it to the love stage and those three words are wasted and feelings can be hurt.

Play it safe

Some people would prefer to play it safe and not say "I love you" to someone until they are pretty sure the other person loves them back. This is ok to do as long as the person who wants to say "I love you" is ok waiting for as long as it takes for the other person to love them.

Next level

Saying "I love you" generally takes a relationship to a new level. Because this is usually the case, a person should not say "I love you" until they are ready for the relationship to go to the next level. The next level usually means even more commitment and for many people it can mean an engagement is in the future with marriage soon to follow.

Romantic

A person usually does not need to go to great lengths to create a romantic atmosphere for their partner before they say "I love you." Usually the act of saying "I love you" creates the romantic moment all by itself. But it can't hurt to plan it out a little bit or create a quiet scene so that the moment is even more special.

Just the two of you

It is a good idea for the person saying "I love you" to say it when the two people are alone. This can be a good thing for several reasons. For one, it can make the moment more romantic. And for two, if the other person does not feel the same way the person proclaiming their love does not have to be quite as embarrassed as they would be if they said "I love you" in front of family and friends.

Timing

Sometimes people are able to feel love for someone after only knowing or dating them for a short time, but for most people love takes longer to develop. It is probably a good idea for a person to wait a while before saying "I love you."

If they have only been dating for three weeks it can be hard to know if the feeling they feel really is love or if it is infatuation or if they just really like the person. It usually does not hurt to wait a little longer. Either the feeling is still love and will get stronger over time, or it was never love and then it is better that they did not say "I love you."