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This looks to be sunburn or sun scald on the leaves.
I would suggest a more shady location for these darker green leafed Hosta.

https://www.gardeningknowhow.com/ornamental/foliage/hosta/growing-hosta-plants.htm

There are too many factors to give any kind of number to this answer. Generally, you will want to water for 30 minutes to an hour whenever the soil is completely dry down to about 2 or 3 inches.

This can depend on many factors, such as temperatures, humidity, rainfall amounts, and so on. The best way is to, physically, check the moisture content of the soil. This can be done by hand, or meter. A good soil moisture meter isn't expensive and a good tool to have around.

Other factors include the specific water requirements of each plant, and how much water they need in general.

This article will help you to water plants correctly in hot weather: https://www.gardeningknowhow.com/garden-how-to/info/watering-during-heat-waves.htm

Hosta grow in Zones 3-9 and you are in zone 9b. Plant your Hosta in a full shade location.

If this is not a variegated hosta, and is a green leaf variety, then it is likely getting way too much sun. These do well in shade, and can handle some sun, but they will burn very easily. This article will help: https://www.gardeningknowhow.com/ornamental/foliage/hosta/growing-hosta-plants.htm

Root rot is also caused by a fungus, but it's most common in overly wet soil. The roots begin to suffocate in the waterlogged ground, causing the crown of the plant to become more susceptible to rot and the leaves to begin drying along their margins. Pull the mulch back so the soggy soil can dry and reduce watering temporarily. If the plant recovers, prune off the damaged leaves to improve the plant's appearance.
Root rot is also caused by a fungus, but it's most common in overly wet soil. The roots begin to suffocate in the waterlogged ground, causing the crown of the plant to become more susceptible to rot and the leaves to begin drying along their margins. Pull the mulch back so the soggy soil can dry and reduce watering temporarily. If the plant recovers, prune off the damaged leaves to improve the plant's appearance.
https://www.gardeningknowhow.com/ornamental/foliage/hosta/hosta-plant-diseases.htm

It is getting a little late, and you may have some fail to make it, but generally you can divide them at anytime that they are actively growing. This article will help: https://www.gardeningknowhow.com/ornamental/foliage/hosta/dividing-hosta-plants.htm