Where is the ideal setting for a gas water heater?

Im a new homeowner and moved into a home where hot showers only last 5 before it turns warm. Right now the temp gauge is set to almost midway between warm and hot and there is a line on the knob where it is set to, I don't know what that line means. Would it be ok to increase the temperature, would that give me longer hot showers? The knob doesn't tell me how many degrees it is set to right now.