Thank you very much.
This leads to another question:
If I start as fighter and multi class to mage, does it mean the DM says: "OK, you go to a temple and train there for 3 years." In the meantime the other single class characters continue the adventure, gain many levels and maybe even finish the whole story.
Most GMs simply ignore training, as it would halt the game. Is like a quality of life improvement in a video game, even if it doesn't make sense.
For example playing a table for one year and reaching level 20 with a caster is nonsensical lorewise. Level 20 casters from the lore like Blackstaff and Elminster are centuries old, even in BG level 10 wizards will be very old.
