Answer: Lemons take months to ripen. The only way to tell if they're ripe is by tasting one. If one is ripe, the rest of the crop of the same size and age should be ripe, too. Sometimes they drop from the tree when they're ripe; sometimes they hang on the tree for months after they ripen. A taste test is the only sure way of determining the ripeness.
If your grapevines look sick I'd suggest pruning them off the plant, both to remove any potential disease and to encourage healthy new growth.
Good luck with your garden!
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