Engine Check - 1999 E320 - loose gas cap?

[bossman280]

My 1999 E320 has 114K (kilometers) or about 70K miles.

I've had it for about 6 months and it drives really well.

Last night I filled it with gas and today I noticed that the "check engine" light came on. I took off the gas cap and re-tighted it as someone suggested to do. It may not have been completely tightened but I'm not sure. The car seems to be running 100% but the light is still on. Someone told me that you may have to drive it for a few days for the light to go off.

Is that right and how may days should it take for the light to go off (maximum) if it was only a loose gas cap?

Any other suggestions are welcome.

thanks