I used to break it into 3 categories, but I think 4 gives you a better picture of Tapas overall. And this is Tapas specifically because as @Kelheor said, on other platforms things are different.
Spending time on the forums, it can feel like the big milestones like like 50, 100, 250 and once you're past 250 you're a big deal, but when you look at Tapas as a whole, 250 is really small next to several thousand. 250 is just barely enough that you've unlocked monetisation of your work; it's the point where a work can potentially jump from hobby to job if you work hard, and to climb past it, and especially past 2000, you really do need to take your work as seriously as if it was your job, with polished presentation, effective blurb, good advertising etc. Because past even 250, most of the people who are your peers have at least some professional level training or experience.
And I don't mean it like 100 or 250 isn't an achievement. If you're a hobbyist who has no professional training, or if this is your first comic (even if you're experienced at illustrating other media) or novel and you managed to get a hundred people to read it? THAT'S AMAZING! You did amazing! My very first webcomic had like... err... thirty? 
Like seriously, somebody with no formal training or professional experience in writing, art or advertising it just kinda... threw together a comic or novel while learning as they went and probably some cheap tablet and paint tool sai or something and got 100 people excited for it? That's genuinely impressive. That shows the idea has legs and that person has moxy.
But if you want to break past 250, you're competing against pros and will have to train and work like a pro. You'll need to seek out pro resources, pro level feedback, use pro-level media/equipment or commission pros for art/design work, because the other people you're competing with for eyeballs sure as heck are, and Tapas is a business; they don't put stuff that doesn't look professional on their front page. This might make anything over 250 seem totally inaccessible to most people, but the resources are out there if you're ready to put the time and research in. Getting feedback at that level and changing your approach can be rough, and it takes a lot of time and work to build these skills, but anyone can do it with time and effort.
I break Tapas levels down like:
Level 4: 0-250 subs. This is either a new work or a work that due to either not appealing to the Tapas market, not being widely or effectively advertised, not updating much or not being polished in quality can't climb beyond this level. Works of this level make up probably the vast majority of works on Tapas, so most works by people on the forum are in this zone.
Level 3: 250-2000 subs. A work that has broken through the 250 mark, assuming it got there through organic subs, is pretty likely to get staff picks and front page features because it's usually decently polished. Some works that are fairly popular and polished examples of their genre, but very niche might stay here for a long time or even indefinitely, but due to appearing on the front page, most works that can get to this level, if they keep going, tend to keep growing. At this level, a work probably makes the creator a bit of pocket money to withdraw now and then but not anything to shout about.
Level 2: 2000-10k subs. Works of this level are big enough that creators can pitch a premium series. They're big enough to generally successfully fund printed books and similar on kickstarter and probably make the creator a small but regular supplementary income outside of their main job.
Level 1: Over 10k subs These are the big works on Tapas that tend to dominate the front page. Making a work of this level might well be the creator's full time or part time job.
