In this case, going from a 32GB M1 Pro MBP to one with 64GB is effectively a $600 (or more) increase (because it forces getting extra GPU cores, which are of no benefit for LR and PS)...
The short Answer:
How much RAM someone should get is just so subjective and depends on many factors, not the least of which is how someone uses a particular piece of software. In the case of Photoshop; photographers who mainly focus on color correction and touchups won't see much benefit going from 32GB to 64GB of RAM. But designers/photographers who "build" images using tons of layers, run lots of filters, etc. will absolutely see benefits.
In the end, there is no straight right or wrong answer for the OP's use case. Though I will say this, if he's keeping these Macs for 6-10 years as he says, the cost of 64GB RAM upgrade is trivial. Even if kept for 7 years (84 months), a $600 upgrade comes out to around $7 per month. I'm sure he'll cover that cost before he finishes his first cup of coffee on the first day of each month.
In reply to your comment:
I don't use LightRoom, so I defer to you and everyone else with regard to how it runs. But Photoshop uses the GPU for screen redraws... having more GPU absolutely has a benefit in Photoshop, as does more RAM. I'm also in the camp of "if you have money to upgrade, spend it on RAM (not storage)" because you can add storage externally, you can't add RAM.
I work on 1-2GB Photoshop files with dozens upon dozens (even hundreds) of layers, along with having InDesign, Illustrator and Acrobat open. I've gone years with 32GB of RAM in Intel-based Macs. If I were to put a word to it, I would say it was "adequate" for most work, however, working on an Intel Mac with 64GB of RAM was MUCH more "fluid." If I were going to buy an Intel Mac, it would definitely be 64GB of RAM.
Here's the tough part, I recently upgraded the Intel-based MacBook Pro 16" with Core-i9 processor and 64 GB RAM (I forgot the video card I had in it, but it was the highest upgrade available at the time), to a Mac Studio with M1Max and 64GB of RAM.
In my opinion, it's still too early/difficult to tell where the massive performance gains are coming from for me (in comparison to Intel Macs) with regard to Apple Silicon-based Macs. Using Photoshop and Illustrator (which ran like a slug on Intel Macs with complicated vector art) is lightning fast now. I've used an M1 processor Mac with 32GB RAM and it was already like night & day compared to the Intel, but the M1Max w/64GB RAM is almost indescribably faster. I don't know if it's JUST the M1Max and faster SSD, or JUST the RAM, or a combination of both. Either way, my workflow has seen a very obvious benefit from having more RAM (along with the much faster processor).
In all cases, I run a fairly clean system. I don't run a ton of menubar apps, background apps, system extensions, or frivolous utilities. I believe that can make a big difference as well.