When it's about the size of a goose egg, or smaller. If you leave it until it starts to go yellow, or it gets too big, it will lose the 'cucumber' taste. You pick cucumbers as unripe fruits of course.

Thats interesting,Bertiefox,as my friend gave me one of hers last year,and it was yellow,about as big as a grapefruit and tasteless! So thats the reason..I was a bit of a coward and said it was nice ,when asked,as she was so proud of them.!!.As she was on a visit from Lincolnshire,and I live in Yorkshire,,no further gifts of these were forthcoming on a regular basis..I would have hated to hurt her feelings,as it was a kind thought....