there is no time line to saying these things I think if you do miss her say it in a more tonful dominant way so instead of saying "i really missed you" you could text her while she's walkin away from you somthin like "waw i see that tush movin left and right away from me and i already feel like i'm missing you :p" putting i miss you in a sexual and humourous way shifts the situation from power struggle to what we call an exciting relationship. now what i gave you above is just somthin to demonstrate what i mean, if you do feel like saying somthing and your not really sure always put a twist on it either sexually, humourous, or even a small neg at the end of the line to loosen things up. girls don;t like guys that come from a scarcity point but a place of abundance and establishing that with her constantly by playing a role of alpha male would gaurentee a response back from her, this can be good for long term relationships also; as it tends to spice things up. a piece of advise for the futre.

good luck buddy

If you did miss her, why wouldnt you let her know?

It is a difference between being needy (like making that the only thing you talk about) and being honest and sweet. "Its kinda strange, we have known each other for not very long now and today I actually missed you, we always have so much fun together, you are now officially in my inner circle!"