People have individual stat limits. Also, you gain excelia based on what you are doing, so each run would vary.

If a days expedition is a single dungeon run, that is less than two years to max out your stats.

Most people should be like Lily and struggle to have even one stat reach 500 after a decade or more. I say this because Omori intended for Lily to be representative of the average adventurer.

So, I would assume the answer is no for most people.

Lili was actually meant to represent "those without talent" and she isn't a good comparison for an average adventurer. It is directly said that as a supporter she gets less excelia than them. Also, ten years is just too much to reach rank D unless the person in question was inactive for long periods of time.

As for examples, Bell had gained a total of 331 points in about half a month before he got Liaris Freese. There's also Welf, who in around seven years since he met Hephaestus accumulated a total of 2362 points, even with smithing being his main priority and not getting that much help from his fellow familia members.

But there's also some factors to take into account. The growth curve, meaning smaller increases as stats are higher, and the support of one's own familia. One's familia matters a lot because if there are members of higher levels and willing to help then the newbie is going to grow quickly, and there is also the matter of excelia distribution when going in groups. And on top of those there are also the ones Rqwe1 mentioned.

I made it so that there is a decay of 2% of the stats gained by each consecutive dungeon runs (the first dungeon run receiving 40 stats (as an absolute noob) so the second dungeon run would be 98% x 40 and the following run even smaller than that.

1-30th run = 2% decay (multiplied by 98%)

30th-60th run = 1% decay (multiplied by 99%)

60th-100th run = 0.6% decay (multiplied by 99.4%)

100th-the rest of the run=0.5% decay (multiplied by 99.5%)

The result was roughly 613 runs are needed to get max S999 stats on every category (at the start received 40 basic stats and at the end (613th run) received 2 basic stats).

If my character were to do that everyday for 2 years straight without any setbacks I would think 2 years sounded plausible to be honest.

FYI. this is for a self-insert fanfiction im doing (MC is a college student from the modern world with knowledge on nutrition and psychology)

Have fun and good luck with inspiration while writing,