Too many factors and unknowns for anyone to give you a specific quantity and frequency of watering. Will also change with the weather. The goal: Water thoroughly until water comes out the bottom pot then let dry somewhat before watering again. Typically, you don't want to wilt too hard a hydrangea, espcially in bloom.

Good advice! The frequency/how often will depend on the size of the size of the container, the potting medium and the weather but you DO want to water thoroughly when you do. Until the water runs freely out the drainage holes. I wouldn't rely on a moisture meter, either......they are not all that accurate. Test the soil with your finger and if it is dry the first couple of inches down, it's probably time to water again.

FWIW, we water daily at the nursery and in hot, dry weather, often more frequently than that. But nursery hydrangeas tend to be in smaller containers than those grown at home.

every other day or daily if hot--like now-headed to 100 tomorrow.....

Here in Houston, we're having record heat (up to 105 a few days ago) AND we're in the middle of a -18" drought (worst since 1956). I'm watering at first light and right after sundown during the week. On weekends when I'm home I'll give a mid-day watering, as well. Leaves are STILL brown and crunch and hardly any blooms are intact on my Nikko, Mdm Emilloire, Masjas, T&S, ES and Nantucket blue, which get a lot of sun and the last 3 are in pots. The ASB, Trophee and Oregon Pride which are in the ground and get less sun are doing fine (though all their blooms are white or nearly white... not sure what's up with that).

So, depends on the situation. It's not possible to overwater them right now where I am.

One more factor- a way large root mass in a small container will dry out far faster than a small root mass in a larger container.
How large are the containers, how large were the root masses when you planted them, and how long ago did that take place?
:)