1 3/8" is typical.

I've got a faucet and separate side sprayer. They're 8 inches apart, center of hole to center of hole.

You want 1 3/8", not 1 1/2". Some hot water dispensers require a 1" hole maximum.

Read this after he's done drilling, please.

@ friedajune: Thank you for your suggestion. Yes. I do live in Wisconsin. Our kitchen remodel did not involved with changing layout, just simply repaint cabinets, replace old countertop, add a few more outlets, changing floors, but we do manage to make a room to add a dishwasher. So basically it is a DIY project, except we hire an electrician to do all the wiring. We are not so sure if our village require air gap for a dishwasher, but all the countertop stores we visited said we will need a hole for air gap, so I would add the air gap just for passing the inspection just like you said. We may use the hole for something else in the future, like an air switch for disposals.

I'm thinking to have the spacing at 6 inch apart...

Don't get the air gap if you don't need it and counter top stores are not the authority on the subject. Its one less thing to have to clean around. I'm really glad I put in a high loop instead of an air gap, the only hole in my counter is for the faucet.

For the main faucet, I suggest 1 1/2" which is the minimum for a Waterstone faucet. Don't put yourself in a situation where you eliminate the easy replacement down the road with an upgraded faucet. The other holes can be 1 3/8" in my opinion.