Took Enclave to dealer for injection cleaup

Some injection cleaning systems use a can of pressurized "soap" attached to the fuel system service port while the fuel pump is disabled. That means the engine isn't actually running on gasoline for about 20 minutes. To achieve the correct fuel / air mixture, the computer makes changes to how many milliseconds the injectors remain open during each pulse and that's going to be significantly different than when it's running on gas. It's the length of those pulse-widths that your fuel mileage computer uses to calculate fuel mileage. Also, the vehicle was standing still during the procedure so it was getting 0 miles per gallon.

The average fuel mileage is going to be very low for a while. There are also numbers in memory called "fuel trims". Those represent how much extra fuel the computer needs to add or subtract from the factory-programmed values to achieve the correct mixture. Those numbers will also have changed a lot. They will update automatically as you drive. You should see the fuel mileage gradually improve as the Engine Computer continues to readjust to running on straight gasoline.

Impossible to say. Could take a few days or many miles. The Engine Computer is constantly readjusting the fuel / air mixture for best performance and lowest emissions. As long as the system is working properly, fuel mileage is mostly a product of vehicle weight, wind resistance, and friction from the brakes and wheel bearings. Not much you can do about those things.

Fuel blends change this time of year too. If you filled up for the first time in California, you'll automatically get about 2-3 miles less per gallon just due to the gas. Adding ethanol also reduces fuel mileage by about 20 percent.

I'd pay more attention to the gas gauge for a while. Don't even look at the fuel mileage reading for a few days. If the mileage doesn't come back up in a week or two, your mechanic can connect a scanner to display live data to see what the oxygen sensors are seeing. If they're switching between rich and lean a couple of times per second, they're working properly.

What kind of problem were you having that required the injector cleaning? If they were partially plugged with carbon, they may have not been flowing enough fuel before. A lean condition results in hesitation, stumbling on acceleration, and increased emissions.

It may also just be that the injectors are clean now and spraying more fuel. The 0/2 sensors could be clogged and not recognizing an over rich mixture.