There's only a handful of zones in the current version:
Outer sanctum, home of basic slimes. From here you can go to the inner sanctum, catacombs, or swine country
Catacombs, home of the undead. The game's only dungeon currently, but there's three levels and each level has multiple variations
Swine Country, home of mekkboars. Don't know the official name of this zone, but there it is. Used to have another zone or two after, but those have been blocked off due to not really having anything. Even now it only exists as a place to farm mekkboars for the repeatable quest and a unique drop or two if you care
Inner Sanctum, home of slimeks. Has a unique mechanic where killing enough slimeks summons a boss, leads to the ruins
Ruins, home of golems. Touching two sigils here brings you to the end of the current main story quest, but you'll probably only be somewhere around level 10-12 when you do so. Afterwards you're expected to return to:
Catacombs part two (and three), home of the good loot. Your first dungeon run is rather basic, but the lv6-12 and lv12-18 versions have optional bosses at the end with a bunch of unique drops and ramps up the difficulty a fair bit, giving you a reason to actually think about your playstyle
Level cap is currently 20, and there is technically gear with that high of a level requirement, though at that point there's nothing to really use it for (yet). As a thief that's plenty of levels to get all the skills you need (since it's split between katars vs bow, which scale off different stats so you're not going to want to try to use both on the same char) but warriors and especially mages have a lot more cross-compatibility with the skills in their respective trees which makes picking your levels more important. You may even want to sometimes hold onto a couple points to meet the level requirements of some skill ranks.
You can beat the unique quests in an hour or two, but getting to the point of beating the strongest boss will take considerably more time, and if you want to actually max out a character you could be looking at ballpark estimate of 6ish hours depending on how efficiently you're grinding and how lucky you are with equipment dropping for the character you're currently playing as (though mimic chest carries over between characters, so if you're playing a warrior and get an epic staff drop, you can toss it in there and use it on your mage character once you get one to a high enough level)