Cucumber slices will keep in water and in the refrigerator for 1 to 4 days before they start going bad.
If you're using the cucumber slices to make cucumber water you can leave the cucumbers in the water for 12 hours which is enough to make cucumber infused water.
Then the cucumber infused water can last up to 3 to 5 days in the refrigerator.
However if you're just using the water to keep the sliced cucumbers in the water fresh then they should last for up to 4 days in the refrigerator.
Be sure to keep the cucumbers that are sliced in the refrigerator until you're ready to eat them or wait to slice them up until you're ready to eat them.
Cucumbers that are not sliced will usually last a week or more out of the refrigerator but once you slice the cucumbers and put them in water they must be kept refrigerated to keep them from going bad.