1-85 takes 20 minutes including all class changes
85-105 takes from 5-6 hours (gear+pots) to a couple of days (no gear or pots)
105+ if we are being honest requires you to spend RL money , there are no drops of any use to you in game, you can't farm enough adena to buy anything till 115+
IMO as a baseline, to progress with a semblance of expediency the minimum you should expect to spend is $200/month, obviously the more you spend the faster you progress. I realize that sounds expensive, but that's reality. You have to catch up to what everybody else has been doing for 10+ years. The smart way to spend that money is to buy whatever "sales" item are available (we 're talking gacha boxes) and DO NOT open them, sell them ingame to other players. Use the adena you earn to buy items for your character. That will probably change next week, with the introduction of a new premium currency (einhasad coins) which are tradeable and can be sold directly for adena.
Easiest/cheapest class to play is Eviscerator (Ertheia Fighter). Needs the least amount of gear BUT is limited exclusively to single target (kills one mob at the time). Second cheapest character to play IMO is Maestro (Dwarven Fighter). Both Eviscerator and Maestro have a skill that has a chance of removing 30% of a mob's hp with a hit. That's what makes them cheap to play.
Faster progressing classes are aoe classes (kill multiple mobs at the same). But these need a lot more gear. Currently Death Knight (Human male special class) is the most popular class, but don't expect to be able to play it with the free stuff the game gives you. Other options are Mages (will get boosted with the upcoming update on August 1st), Dreadnoughts (spear users), Archers. All these require A LOT of gear (meaning a lot of money) to be played effectively.
You are currently allowed to have 3 Lineage 2 windows running at the same time. Make two isses (A hierophant (human mage) and a Sword Muse [elf fighter]) party them with your main and make them follow and gain exp after 85 (everybody can get to 85 easily solo). You get Free buffs until level 99, after that the 2 buffers will be your main source of buffs/heals and no you can't play this game without buffs. Population is low. Expect to find parties easily from 100-105 (blazing swamp) and 105-108 or so (silent valley) but if you choose that route, you will find yourself at 108 unable to even kill a mob solo if you don't have buffers.
When you complete your final class change at level 85 you get a free vitality maintaining rune for 7 days. Take advantage of it to exp quickly to 105, and exp your dual class quickly to 105 too. You will gain skills that are pretty useful once you do. After that, you grind. Expect to spend most of your time ingame alone. Basically, to be honest, the game now pretty much runs in the background 95% of the time, you just live play for a few minutes (at 105) to a few hours as you level up doing your dailies. Yes the game includes a botlike function that allows you to auto kill mobs with the game running in the background while you watch cat videos on youtube.