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When are they ready to be picked ?

Never planted them deliberately, but have a thirty foot by ten foot
volunteer in a grassy area.

The fruits are quite large, and i've turned them to eliminate the yellow
patch on the earth side,
but none of us know how to tell when it's time to harvest them.

TIA
If you wish to use them for winter (baking squash) just leave them until the
vines start to die. Actually you can pick them fro this purpose as soon as the
stem becomes dry an woody. Some folks also use them as summer squash in which
case you pick them before the skin becomes hard.