when to remove propagator lid?

Hi all, a silly question that I have been dying to ask - when is the best time to remove the lid from my unheated propagators? Do I remove it when all the seeds have germinated or do I leave them covered until they are bigger (and if so how big?). I know seeds are determined (usually) to grow and whatever I do some will make it and produce fruit and veg, but I am sure that if I do things properly I will get far more plants and far healthier ones too. I suspect, out of fearing they will get cold (my house is generally freezing!) that I have been leaving them covered for too long. I would really appreciate your advice and opinions on this. Thank you.

Ah thanks DD, I have definitely been doing it all wrong and explains the quantity of leggy plants I get!

thanks for asking this it helped me too