All spellcasting classes gain expert proficiency in spells at 12th level, master at 16th, and legendary at 19th levels. I feel like they should have these proficiencies granted to them at earlier levels, because I've noticed that my spellcasting players' characters have had lower than seems reasonable DC's for their spells. Also, these spellcasting focused characters have the same level of proficiency in using magic as anyone else who gains a cantrip or other spell through innate magic up until level 12.
I'm not sure what levels seem reasonable for these proficiencies to increase, but I would probably consider a character that can cast 4th level spells to be an expert, 7th level spells to be a master, and 9th level spells to be legendary. This would mean being expert in spells at level 7, master at level 13, and legendary at level 17.