rpg

The entirely unsatisfying answer is, "As much or as little as I have to."

I haven't run a 5e adventure, but I imagine that would require a LOT. Part of this can't be helped, because in any long campaign, you're going to have to be more flexible to account for the unexpected and accommodate your players. The other part of it is that D&D's design philosophy seems to be "eh, fuck it, if it's broken, people will just blame the DM and say it's their responsibility to fix it." A part of this is the entire CR system (in a game that focuses so heavily on balanced combat encounters, no less), so I'm not surprised that tinkering with combat counters is so necessary and so frustrating.

I find Paizo's Adventure Path line of products to be really good. I frequently tweak encounters, sometimes brewing my own NPCs for them, and occasionally add my own scenarios to bulk them out or elaborate on something that wasn't detailed in the AP. The actual stories I don't often touch, however. I've yet to really need to.

You pretty much have to, in my experience. The monsters are often optimized for min/max players, and if played intelligently will wipe a group focused on playing as characters with no knowledge of what they're fighting. Also you always have that one player that reads the adventure (one offered to send me stats for a creature I had interacting with the party!)

I've been using the Pathfinder 2E Beginner Box, and it's the first time I've run a premade adventure. I've been customizing it quite a bit in terms of the story to better match my players. I expected the adventure to feel stale and on-rails, but what I found was that it gives you a safe baseline to work from. If you find any parts of session planning stressful, you can just leave them at the baseline and devote more of your time toward the things you actually enjoy.

In my case, I was still learning the system, so it was nice not having to worry about balancing encounters, drawing maps, or distributing treasure. Instead, I was able to spent my prep time on modifying the story.

I am returning to playing RPGs for the first time in well over a decade this year. I ran a few modules in my first life as a rpgista, starting with the adventure that came with the red dragon black box of Basic D&D. Later, I ran the introductory adventure that came with DragonLance SAGA and Sunless Citadel, for D&D3. But I ran them without modifications.

Now, many years since I last did any playing or gming, I am running "The Lost Mines of Phandelver", but I am changing quite a bit of it. Mostly because of the dislike we have for what was done to Forgotten Realms after D&D4. So, I pushed the adventure back to 1368 DR, what required some changes. Some of the ruins weren't ruins in 1368 DR, so I had to change places or repopulate other places using my old AD&D2 and D&D3 books for Forgotten Realms. It was more work than just running the module straight out of the box and even making my own homebrew adventure on my own homebrew setting.

Despite the extra work, that I welcomed after so many years without RPG, I am finding it very gratifying making that module my own, and I am already preparing to continue the campaign organically, without a hard cut. Hopefully, nobody will notice when I change from the published module to my own stuff in the campaign.