At startup engine requires rich air-fuel mixture so slight accelration is required... you'll notice that all efi cars have auto-choke function that accelrates engine automatically at start-up and remains accelarated till engine gets required optimum temp.
For auto-choke engine, there is no need to accelerate it manually. Engine will automatically maintain its temp. You can shut down when temp gauge touch middle.
Incase ur car is carby or without auto-choke then give slight constant acceleration after startup not beyond 1500rpm till the time engine temp starts to rise. Once temp start rising you can leave it idle till gauge touch middle.