Interesting. How do you go about tracking this, or gauging when it's time to level up?
Good question.
Some factors - our table moves at a slower pace than most, high level 13th, I do not keep track of XPs but rather Inspiration points used.
We have 3 level-up conditions at any time...
(a) Completion of 1/2 Level+1 (i.e. 7) Sessions and the use of Level+1 (i.e. 14) Inspiration points (milestone and XP variant)
OR
(b) Obtaining a Conch of Teleportation (achieve a great feat in the storyline)
OR
(c) Attending the 4th Council Meeting (timeline event)
As soon as they go up a level, the slate is wiped clean for (a), and (b) and (c) are updated.
The point for this entire exercise of mine was more to ensure
(1) The PCs integrate with the narrative aspect of the game
(2) I have a levelling-up system that is player-facing and that works with my campaign pacing.
So, to answer your question I analysed my campaign and used numbers that best suited it based on our play history & frequency as well as how the campaign story would progress and what I thought would be fair. I guess the earning of Inspiration points is tempered by the number of sessions needed (milestone equivalent).
I'm running Tomb of Annihilation, and the PCs have recently, finally entered the titular tomb. I quickly realized that encounters were going to happen much more frequently there compared to the game thus far, so now I'm allowing long rests every 8-10 encounters instead once per day.
However, reading your comments makes me think I'm going about this the wrong way. I know long rests is different than leveling, but maybe I should be focusing on character arcs and story hurdles rather than encounters.
Yeah, I see your dilemma.
I haven't purchased
Tomb of Annihilation just yet so I cannot comment insightfully on progression within that storyline but I will say that I'd imagine my current campaign which is a ToD/SKT mashup allows for much more freedom for characters to play into the BIFG which doesn't help you.
However - you can use your pre-existing pacing metric of 8-10 encounters which is a milestone mechanic essentially and tack on x required Inspiration uses. Obviously not too many if it is difficult to leverage their BIFG within the Tomb.
I hope I've made some sense in all that rambling.