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 have never used a campaign module as written. I don't know that I could. I started with homebrew adventures, and that's what my mind adapted to.

I get an idea of the adventure module in my head as I read it, and then I fill in the idea by myself. I often think over, adapt, expand and rewrite aspects of the idea, making it something I feel I could portray to my players. Then I use the original adventure as reference to fill in the gaps during gameplay. The end result is my version of the adventure, which is like the same image drawn by a different artist: recognisable, but different in obvious ways.

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.

I had to tweak many individual encounters to match what I wanted. So running it requires looking at the adventure and looking my notes for things to override.

I customize commercial adventures regularly and pretty extensively. I find buying and running the adventure in some digital format to be very helpful. If I buy a Foundry adventure, editing everything is a built in feature. If I buy a PDF, I copy text over from the PDF into my own digital notes and edit them. This way, I end up with a fully integrated adventure with whatever I want to pull from the commercial text integrated right along with whatever changes I want to make.

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.

In the past I’ve always run entirely homebrew adventures and settings, but I recently started Waterdeep: Dragon Heist to save some prep time. The campaign is okay overall; it’s great to have an existing setting with decades of existing content if needed. I’ve found myself having to redo or add a lot though: the campaign was obviously designed to be run for 4-6 players, so with my smaller party of 3 I need to come up with more encounters and rebalance what is there. I also can’t stand some of the tropes they use and re-fluff them to be more dynamic.

I don’t think I’ll do a published adventure again. I’m open to existing settings now, but the style of published adventures just doesn’t vibe with me.