Never gone over 8 days battery since day one

I have a Fenix 6x Pro and I mostly get about 5 days with my usage (100% to less than 10%). For me that is normal considering that I have all sensors enabled (pulse ox on all day). When looking at my gps usage it is normal that it reduces the estimated time of 10 days to around 5. 

That looks very limited, but when running beta firmware I tend to check the device usage widget to get a good indication on how long for example gps is used. When doing the math with all the settings I have and considering I use other maps I can't complain about the battery life.

The same was true for my fenix 5 plus, which lasted only a couple of days, but fully in line with specifications.

I am using the watch for about 8-10 hours a week in Gps Mode, and 1-2 hours Tracking without GPS. I also have pulse ox in all the time. For me this ist great.

Bluetooth ist Off 90% of the time though, because of Home Office in the current Times. 

Charging a Fitness tracker only once a week is really good i think. 

I JUST put my watch on the charger, first time since it auto-updated to FW 16.60 (yes, the recalled version) a couple of weeks back. My 6X Sapphire lasted 19.5 days on the new firmware, going from 100% to 1.2% battery. Which is 1-2 days longer than on 15.20 (probably within the margin of error).

My settings:

• Backlight - gesture-on during sunset hours, 20% brightness, 8 seconds.
• GNSS: GPS + Glonass
• Wifi off
• Pulse Ox: off
• HRM: on
• Activity tracking: on
• Power Saver - on during sleep (HRM + 24/7 activity monitoring enabled, so it's just watchface, DND, and BT off)
• BT: on and always connected to phone (except during sleep)
• Stock digital watchface

My usage:

• On-wrist 24/7
• ~15 GPS-enabled activities (~17hr 30min). Using HRM-DUAL and RD-POD via ANT+

So while the new firmware may have some issues for people, it's been quite good for me. The only odd behavior I've seen was the watch sometimes going into power saver mode after exiting an activity (which happened on the old firmware too).

20 days is estimate for 6X which has a larger battery. Why do you need Pulse Ox? It isn't accurate anyway. I've never used it in year and half I owned the watch. Sometimes people forget that this is a GPS watch first and foremost. All other features are extras. It seems Garmin is starting to forget that too.

I think your experience (and mine) are pretty closely aligned. IIRC, Garmin rates the "21 days" as including 1hr/day of GPS activity tracking. So if you aren't tracking that much, should be easy to meet/exceed the number.

I'm very interested in seeing how much Solar stretches it. I'm getting very spoiled by not having to worry about charging the watch for 3 weeks at a stretch. 

Personally I think it is unhelpful when X owners chime with their battery stories in on a non-X Pro thread. I think this is why the poster has unreasonable 20 day expectations.

I don't feel like I was being unhelpful, I clearly explained in my first point that one of the primary reasons he's not getting 20+ days, while some of us are, is because the 6 is not rated for it, while the 6X is. Like you said, perhaps they had the unreasonable expectation because they weren't aware of that difference. I didn't point it out in a rude way, and I explained various things they could do to increase their battery life.

But it seems clear that the OP was unaware of the different battery capacities between the watches. Otherwise the OP would have referenced the 6 specifications.