If a person you're dating says 'I love you' and you're not quite there yet, what good can honestly come from telling them that? Their feelings will be hurt and it will only make things awkward between you. Whereas if you say it back, even if you don't mean it, the relationship will go one of two ways: either, you'll stay with them and eventually you will come to love them (so does it really matter if you told a little white lie earlier?) or you'll break up with them, in which case it will be irrelevant anyway.
You really shouldn't unless you actually feel that way. If the person says it to you, you shouldn't be subjected to say it back unless you feel that way also. If you lie, saying you do love them back that's technically leading them on. And it really doesn't matter if you say it back to the person because their feelings wont be hurt, or at least shouldn't be hurt cause they need to understand that everyone moves at their own pace and can't force love.
Lying at such an early (or any stage) is a bad idea. Even if your partner doesn't notice your insincerity and distrust you thereafter, you'll have to carry on maintaining that lie until you do feel that way.
Honestly and frankly tell the truth: "I think you're amazing and I'm definitely on my way there but I'm not ready to say it yet" if you feel that way. Just be honest rather than build a relationship from a starting point of dishonesty.
Even if they're hurt, they'll eventually come to respect your honesty: I've been on both sides of this and can tell you that honesty is best.