I very much agree with Odyssey - it is a spectrum, and there's not an end point to it either - my relationship and closeness to my DD's gets stronger every year, but who knows where it'll move in future. I'm not sure there is such a thing as 'fully' attached. I mean, what does 'fully' mean? Is there a limit to your love? We all express love differently, is one way of expressing it more 'full' than another? Dunno, my gut says 'no', especially to the idea of their being a limit to how much you can love someone. DD2 is attached to me, even though she has many insecurities, but I dont think I would describe her as less attached than say, DD1, it's just that one is more insecure than the other, and they have different ways of expressing their feelings.
Okay, I'm off on a tangent now and not really trying to answer the question!
Attachment is an enduring emotional relationship. Of course, it's a bit subjective what 'enduring/long term' is, and it depends on the childs age, but whatever good signs you're seeing, it's not 'attachment' if you're only 3 weeks into placement, although of course your child may well be forming a bond to you, which is great.
And of course people's attachment styles (their patterns of behaviour and emotions) are all different, which makes it even more subjective! Two children who have both formed attachments to their parents may not express their feelings in the same way.
I don't think there's a point where your child is not attached and then the next day 'attached'. Human relationships grow slowly, build up over time.
So looking back, subjectively I decided that my children had formed attachments to me based on quite subtle things as well as more obvious things. There was seeking me for comfort yes, and my comfort actually working, that's a very good sign. As is looking for you in a crowd. If your presence helps you child regulate themself and calm down, that's a good sign. If your child had any behaviours which are lessening over time, that's a good sign they are feeling more secure. Wanting cuddles etc from you is good (although personality plays a big part in how cuddly and tactile you are, and it's not necessarily a problem if your child is just not naturally a cuddler)
When DD1 said she loved me 5/6 years in, it felt genuine, and I can't expand on it more than that! When DD2 shrieked mummy and grabbed me for hugs right after she moved in, that definitely didn't feel genuine. But a few years in, it felt different. I'm sorry, that might not be helpful, but I think you can sometimes sense these things.
If i look back at photos of DS, there is a definite difference in his expressions, in his eyes, from soon after placement, compared with 6 months in, and 1 year in he's a different child. And he was definitly attached to me by that point. All the signs, comfort, regulation, wanting me, saying he loved me and it was so real and heartfelt, etc etc.