A player in my game wants to train some commoners to be a level 1 character. How long would this take and how much would it cost?
By the Books
There is no answer to this question, either RAW or RAI. NPCs are statted exactly the same as monsters- unlike in Pathfinder and older editions of Dungeons and Dragons, they have no system for progression in the rules.
Hence, they do not have levels in character classes.
If you decide to house-rule, consider the following rules for adventurers that will come into effect here:
- The PC must pay the living cost of the NPC while they're training, unless the NPC can pay it on their own. The details for this are given in the PHB.
- It is presumed that your character has some sort of history that lead them to become a Fighter, Wizard, or other PC class. This implicit assumption would likely also apply to any NPC who is about to become a member of a "heroic" class. The exact details are strictly up to the DM here.
My Opinion as a DM
Aside from the logistics, I would be extremely wary of a player wanting to do this. I can see three cases:
- The player wants to have additional characters to roleplay.
- The player wants to abuse Leadership shenanigans that often arose in 3.x games.
- The player wants to have a bunch of heroes running around the world.
Case 1 can be fun with the right campaign and the right group. Think of it as an ensemble cast game, where not everyone goes on every adventure (see Firefly, for fiction in this style).
Case 2 will lead to major problems in your campaign. Monitor this very carefully. Building up a follower base can be used to extend the functionality of one's character.
Either way, be very careful with this. Gaining followers undermines several class benefits, such as the entire Beast Master archetype, and the Warlock's Pact of the Chain. NPC followers have been traditionally abused in previous editions. Remember that an NPC adventurer will want to adventure, and adventurers are not generally followers.
Case 3 is the situation I would outright disallow. There's no benefit to the added headache.
The D&D GM basic rules indicate that there are NPC classes, and gives the experience values of monsters by CR. An NPC commoner should already have a level (to give it stats) of a commoner class. Levelling this commoner should be as simple as taking them out with some gear to beat some monsters.
That said, from a lore standpoint this is far from satisfactory. Characters spend years of their lives getting the basic skills of their class, lore-wise (anywhere from a few years to nearly a century, depending on your race and the assumption that they spent a good portion of their starting age developing their skills), and would probably find a flavorful workaround to be beneficial.
I recommend an 80's montage with basic skill training taking a few weeks, and some minor hands-on learning. The NPC classes are generally less powerful than PC classes in versions of D&D, and this can be handwaved as them being less skilled -- perhaps even book-skilled -- and inexperienced compared to the people who meet up at a bar and then go out to save the world.
I put my pants on one leg at a time, just like you. The only difference is that once I have my pants on, I SAVE THE WORLD.