The first 2 (I think) levels of Wanikani move at an accelerated pace compared to the following ones, but on the normal levels, the fastest you came move up is about 7 days (until some levels beyond the 40s that can theoretically be done in under 4 each). There’s a detailed guide to Wanikani here with a lot of info and advice on managing everything.
What speed you should actually go at is going to depend on you, and levelling speed is determined by how quickly you get all of the kanji to guru level (influenced by how soon you do radical lessons because some kanji are locked behind radicals), but that’s the fastest possible.
Yeah, your position shouldn’t ever be lost. That said, if you are going to step away totally (not recommended if you can at all avoid it, but sometimes life doesn’t cooperate), you will either have all of your past reviews you don’t do pile up, potentially into the thousands, or you can activate “vacation mode” to pause all reviews. The latter is preferable, but even then, it’s going to mess with your memory and make you miss more when you return, which will still mess with SRS and intervals and push your items down, potentially sticking you in place for a bit.
You can, should the need ever arise, also opt to manually reduce your level to any previous one.