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.
Saturday, April 14th, 2012 AT 8:35 AM