I share a similar experience to what @Naphthalene and @plantron said: My overall average at all levels was around 24 lessons/day, but during normal levels I’d do 20 lessons a day except on the day where I’d do (most) of my kanji lessons.
Take into consideration that you usually always have vocabulary lessons from the previous level once you level up. The reason why you leveled up is because you guru the last few kanji, and its those same kanji that will unlock vocabulary lessons from the previous day.
My advice with managing leveling up speed (that is not full speed) is very simple: leave 4 kanji lessons and do those 3 days before the day you want to level up. In other words, if you want to level up on the 10th day, do the last 4 kanji of that level on the 7th day.
The reasoning is simple: you level up because you guru at least 90% of that level’s kanji. In 58 levels (except levels 5 and 6), Wanikani has less than 40 kanji per level, which means that 4 kanji will always be more than 10%. This helps controlling everything better so that you don’t level up too soon or too late to your days per level target
Some quick notes:
For levels 5 or 6, leaving 5 kanji to do 3 days before leveling up gives you that 10%.
Remember that the sooner you do the kanji lessons in the level, the sooner you unlock most of the vocab from that level while you’re at that level, which helps to prevent postponing most of vocab for once you finally level up.
By doing the 10% of kanji left 3 days before leveling up, remember that you will still have 3 days until leveling up. This means that having 3 days’ worth of vocabulary lessons for those last days means you’re up to schedule. You don’t really wanna have either 0 lessons or too many lessons to do. It’s a good way to help you understand if you’re balancing things well enough
10 days/level is still very good. I think consistency in speed helps making Wanikani a habit (which is what guarantees that you’ll reach the end of the program without giving up). However, there’s also another perspective: the kanji in the first 30 levels are much more common than the kanji in the later 30 levels. This means that you can go a bit faster on the first 30 levels, and then relax a bit on the speed to allow you more time to study other aspects of Japanese (reading more, grammar, etc).
As an extra perspective, 10 days/level is pretty amazing. It’s level 60 in less than 2 years! I think most people think they’ll take like 5 to 10 years to learn the kanji when they start with Japanese, but it can be much less than that and it’s great
Let me know if I was clear enough