Cucumbers will be ripe when they are ready to pick which is around 70 days after planting them and after they grow.
Between 50 to 70 days after planting the cucumbers they will usually be ready to pick and after pick them they will not ripen so only pick the cucumbers when they are ripe and ready to pick.
All cucumbers that are in stores should be ripe when when they are at the store because once cucumbers are picked they will no longer ripen like other fruits.
So the cucumbers are only picked when the cucumbers are ripe and if you do find a cucumber that is unripe in the store it means it was picked too early and it will not ripen anymore.
A ripe cucumber should be firm and bright to medium dark green in color.
Cucumbers do not ripen once they've been picked off the vine.
If you've picked cucumbers from the vine before they ripened then they are as ripe as they'll get and no amount of waiting will improve the ripeness of cucumbers.
So if you have cucumbers growing you need to wait for the cucumbers to ripen fully before picking them because they will not ripen anymore.
All cucumbers that you buy in stores or at farmers markets etc should be fully ripened because they only pick the cucumbers when they are fully ripe and ready to eat.
Cantaloupes and even watermelons will not ripen anymore either once they've been picked just like cucumbers will not ripen.