M.D.
A., I moved my daughter into a big girl bed at 19 months. She hated her cribb and we went thrue hell every nap time or bed time. A s so as we moved her everything changed for the better. she still sleeps very well.
Every child is diffrent and you will have to take your childs lead. I'm A strong believer if something is not working then you should change it.
As far as how long, I've had freinds were their kids have stayed both crib and toddler beds till 4 or 5yrs of age. I guess it all would depend on thier size. Good luck