if some of them start to go brown on the ends its prob time to get hem down quick
however the yellow can be hard to see sometimes and what i do is take one down and roll it round in my hand and give it a smell
if its gives a nice stong scent then I take mine down as soon as it looks like a dry day
you will get very sticky hands that will tell you you got the yellow lupins in them when you take them down as well
also not all will be mature but the bulk should be .
I am in croyodn and think another 2 to 3 weeks till mine are coming down if that helps