Can duolingo please include a timer for exercises?
Or, could you point me to where we can give suggestions?
You can get a timer in the lingot store when you have 10 lingots. The timer is forever and for all languages once you buy it.
You only use it when you choose to use it - immediately prior to each practice.
You will get option for both timed and un-timed practice after buying this. And based on your preference you can choose either of them.
So far as I am aware, there is no way to disable it, but you can simply not choose that option any time you want to practice without being timed. I use both options regularly.