No. A 20 minute shower is not too long.
I take 20 minute showers myself as I love my nice long warm hot showers.
You can take a shower for as long as you want too.
Well as long as your hot water stays hot enough that is.
Some people take only 10 minute showers and some people take 20 minute and even 30 minute showers if they have a big enough hot water tank.
But if you have an instant on demand water heater then you never run out of hot water.