I have planted a wild cherry to replace the photinia with fereblight. How much watering does it need early on?

Get it out immediately - Prunus belong to the Rosacea family - it will get fireblight. I don't know who advised you that it would be okay, but it isn't.
Just checked back - I can't find a record of anyone recommending a cherry tree in the answers to your earlier question. Unless they did it via private message.

And to answer the water question once you've moved it the tree will need a lot of water!