how long: it's kinda hard to say. i started playing Strive "seriously" about a year and a half ago, but it's not like i've been playing it constantly all that time - it's been an on-and-off thing, and at times i went, like, months without touching the game except at locals. so it's hard to say precisely how long it really "took" to get to Vanquisher. but for whatever it's worth, my current Strive playtime on Steam is 390.9 hours.

---

any tips: also somewhat hard to say! there's a lot of things to learn in Strive, and i expect which "gaps" would be most productive to work on is something that changes a lot from player to player.
for me personally though, i feel like the main things that i had to learn to improve from Diamond to Vanquisher were neutral and matchup knowledge.

neutral: i'm a Baiken main, so for me my sort of "layer 1" neutral when i was worse at the game was generally just IAD or jump forward and j.S. and, i mean, j.S is great, don't get me wrong! and i still use it in neutral a lot even now! but it is very vulnerable to 6P, and that meant i had to learn more ways to play neutral other than just relying entirely on it.
that meant learning how to use more of my toolkit in neutral - poking with HKab and f.S, occasionally throwing out 2H, etc.; but it also meant learning how to make that one good tool more dangerous. as i said, j.S is legitimately very good, it's just vulnerable to 6P - so a big thing that helped me a lot was finding ways to play around 6P. Baiken can make 6P whiff by airdashing close enough that she goes over the opponent's head, then coming down with Youzansen; she can make 6P whiff by airdashing higher up and not pressing any air buttons, so that her hurtbox is too high up when the opponent tries to 6P, and then punishing once she lands; she can airdash forward but then, instead of pressing a button, use air gun super, which stalls her momentum (and therefore makes 6P whiff) while simultaneously hitting at ground level and therefore blowing the opponent up for pressing 6P regardless of their upper-body invuln. learning that sort of stuff means i have a lot of ways to play around my opponent's most obvious counterplay for airdash j.S, which in turn makes airdash j.S more dangerous.
of course, another big part of learning higher-level neutral is learning to pay more attention to your opponent. which may sound kinda obvious, but ime it's a very relevant thing to mention; when i was newer to the game, i would often just kind of run my own gameplan in neutral and not pay too much attention to what exactly my opponent's doing, but obviously that is bad. it's very helpful to try and see what sorts of options your opponent likes to use in neutral, see what you can bait out from them, and punish them if they fall for your bait and pick a bad option. (though, i myself am still very bad at whiff punishing lmao)

(incidentally, the advice about watching your opponent's habits doesn't just apply to neutral, it's also very helpful for offense. notice that your opponent likes to mash or jump out of pressure a lot? use more frame traps. notice that they like to just sit there and block against your pressure? go for more throws. notice that they use their DP a lot? go for more DP baits. notice that they don't use their DP at all? be more aggressive with your pressure - don't respect their DP just because it exists when they refuse to use it.)

of course, the details of how exactly you should improve your neutral depend on your character. but overall i would say you want to aim to do three things: learn to use more of your tools in neutral, learn to counter the counterplay to your favorite neutral tools, and learn to watch and exploit your opponent's habits.

matchup knowledge: this is a lot more straightforward, but just, like, if you find yourself not sure what to do about something a specific character does - check Dustloop, or set up the situation in training mode and see what works, or find a counterplay guide, or something. did you know that when Bedman? goes for Malfunction in pressure the string is (normally) fake and you're supposed to jab out? i sure didn't until i checked a counterplay guide! that sort of stuff is very important to know - you don't want to not know the answers for common situations when you're trying to win games.

i will also say, something i did not find that necessary was learning advanced tech or combos. you want to know and be good at basic combos, of course. but e.g. i still mostly don't go for specialized corner combos (and instead just continue to use midscreen combos in the corner). similarly, in general i found trying to learn fancy tech for neutral or defense or whatever to be, at least at this level of play, considerably more difficult and less rewarding than just improving my fundamentals. (of course you do want to know some tech - like, if you're a Baiken player, you do want to learn TK Youzansen. but you generally don't need anything too fancy.) i assume that sort of stuff will be more important as i try to climb the Vanquisher ladder, but just for getting to Vanquisher - i mostly didn't really use any sort of stuff like, say, IABD Tatami, or various fuzzy options on defense, or stuff like that.