For this how about a time requirement based on the new class you want to take a level in?2) Backgrounds give more oomph, and 1st level class picks give less. Your training time as an apprentice mage is explained by your Sage background, not your wizard class. A 1st level Sage background wizard might know 4-5 spells and have proficiency in Arcana thanks to their background; a 1st level Charlatan wizard might only know 1 spell from their class, but has Stealth and Deception proficiency and some other roguish tricks.
Say to mc into fighter takes a year full time training with a good instructor.
Mc into wizard takes 2 years.
Mc into warlock takes about 5min, a conversation, and a few poor life choices