I DM a home brew 5e campaign for a group that meets once per month for a full-day game. At this point, I'm thinking of just having them advance a level each session without worrying about XP and then designing my monthly adventure for each new level. This will allow me to progress plot lines quicker and have a chance of getting to some fourth-tier adventures without having to wait a couple years. Under my current system, it has taken about a year to get to 6th level. That about 8 hours of play once a month. With the time between sessions, I try to have the adventure (or a major chapter in an adventure) wrap up in the 8 hours, so we don't have to pick up cold in the middle of a dungeon or encounter. So, milestone/session based leveling up just makes sense. It also take some work off my plate. Instead of calculating XP, I can just focus on CR and story building.
That said, below is the current rules for my campaign that has been provided to all the players. I was trying to keep to the XP mechanics and avoid making it to arbitrary. But so much is left to DM discretion that is feels a bit capricious and out of the players control. Increasingly, session-based leveling up seems like the more logical choice.
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HOW EXPERIENCE IS AWARDED
I use a blended approach.
1. Combat/Encounter Based.
This is the XP values of the creatures defeated (not necessarily killed). If a particularly clever use of diplomacy, subterfuge, or other non-lethal method results in "defeating" or avoiding opponents in an encounter, I will award XP based upon the challenge involved. If, compared to a head-on fight, the non-violent or non-combat-victory solutions was very easy, you'll be awarded no, or only a token amount of XP. An example would be that the village asks you to defeat the orgre terrorizing them and you say "no thanks" and head on your merry way leaving them to deal with the problem themselves. You won't get XP for that. If you successfully convince another hero to go in your stead, maybe I'll throw 5% of the challenge your way. See table below:
Alternative is...
XP%
Example
Very Easy
0-5%
You ignore the problem. Do nothing (5% if you arrange for someone else to take care if the issue)
Easy
25%
You sneak past, arrange a distraction
Medium
50%
Political or diplomatic solution. Bold intimidation pays off.
Hard
75%
Elaborate trap or distraction, dangerous and difficult stealth
Very Hard
100%
Exceptionally dangerous ruse. Trap that involves using a character as bait with a chance of great harm or death to that character
Nearly Impossible
150-200%
You convince the chromatic dragon to give its horde to charity and devote its life to protecting the weak.
Again, the above table pertains to traditional combat encounters for which you found non-combat solutions.
For certain non-combat encounters like traps and mazes, a Challenge Rating is assigned and XP awarded based on DM's Guide parameters.
2. Milestone Awards
Rather than come up with a challenge rating for every non-combat encounter, I will set milestones for reaching important non-combat goals. So, for example, rather than award any XP for your various farmstead investigations, after you "crack the case" and/or find a solution to the problem, you'll be awarded XP for successfully completing the mission... even if you had never engaged in any combat.
3. Story Awards
Because we meet infrequently, I may provide experience or level ups to advance the campaign. For example, I allowed some players to roll-up and begin with level 2 characters. If a player misses a couple sessions and rejoins later, I may work with that player on what their character was doing in the mean time and bring the character up to a level where the player can meaningfully participate in the game.
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