What is the most realistic maximum level?

In my questions about a build they said me that 123 is a theoretical level but more realistic is to plan a build around 100.

I've leave the game at level 50 on Cruel and monsters were VERY tough even with my best gear, I think I've been playing casual, or maybe is because I've not trade, and to plan my build I want to know some statistics. How many players have reached 123 passive skill tree points? 50%? 20%? I remember a comment saying that just 5 players have reached the maximum level but maybe it was a misunderstanding.

L100 is almost unimaginably hard. Don't plan for it at all.
L90 is quite a grind, but doable solo.
L85 is a good "generic ending point" for most builds.
L80 and below is easily reachable by doing the storyline and grinding a few Dried Lakes.
If you don't have the "core" of your build set up before Cruel Malachai, you'll likely get brickwalled there.

Here's the current Softcore Talisman ladder:
https://www.pathofexile.com/forum/view-thread/1492621

There are 15 L100s there.

Not sure how much time you have to put into the game, but overall, I'd recommend having the first 70 points routed out. From 70-100, finish off taking all the nearby nice things that you pass by on the way to getting the core keystones and notables. Everything above 100 points is starting to get into "bonus" territory.

I think by 'level' the OP means skill points.

Personally I plan for a character to be finished by L80, which is around 100 skill points. I may bring the character further than that, but the skill points that gives I just treat as an extra bonus.

The answers to all you questions are right before your eyes, in Ladders... 617 Characters lvl 100 in standard.

So level around 80 would have 80 passive nodes +23 from quests. Around 103 passive skill points would be a reasonable goal I think. Thank you.

You can get higher than that with little effort. 85 is the better goal, as 80 can be reached from dry lake runs alone. If you have a large map pool then it should be 90.

80 is when things start to get slow, but you shouldn't give up the moment things start to get hard. Now 85 I can understand, if you don't at least have tier 7 maps you can consistently run. If you have reached the point of having endless tier 7-10 maps 90 is a reasonable end goal. To get higher than 90 is a big grind.

I nearly always stop a little bit higher than Level 90.

If you have a T8 Map pool Base, level 92 is doable in an okay'ish state (2-3% XP a Map). Level 90 is easy even with T6-T7, but this is a different opinion from Player to Player

50, 60, 70, 80, 90, 100
Depend on how good player you are

Depends on how boring a playstyle you want when grinding out those final levels.

If you constantly want to check out difficult content and try your hand on tough bosses with difficult map mods, getting to 85 will be a bit slow and 90 will be a real PITA.

If you skip map bosses that are somewhat dangerous and always reroll map mods that are potentially lethal then 85 is a breeze and you'll hit 90 in no time.