When can i harvest Uchiki Kuris

I grow them every year and I wait until the first light frosts knock back the foliage. By then the squash are as large as they'll get, orange, and should come away easily from the main stem.

Once picked, I let them sit under shelter to 'harden off' a bit before storing.

That works as well, MoS, but I find by leaving them a bit later, the cooler weather helps cool the squash interior so it's better ready for storage, if that's what you plan for them. I usually take in about a dozen big ones to my basement cold storage room and they last ages this way.

It just lightens the load if you thin them out by picking a few that are already ripe

My Uchiki Kuri have grown like triffids, one's shot up the pea frame and the other one's gone 10' across the bed, up the fence and is now wandering along the top of it. Both have put off loads of small fruit, but the fruit's been there ages now and they never seem to get any bigger.  Some of them look pretty orange too, but about 2" across.

Trouble is I daren't put any BFB or tomato food on them now to give them a boost. The've put off so much growth I seriously doubt I could put anything near them without touching the leaves with it. And I've already proved for myself that if you touch their leaves with either BFB or tomato food it burns the leaves quite badly.