A few things I'd like to stress and clarify, in case my wording in the text isn't clear enough:
- Ki-users don't gain levels, they gain abilities. The abilities are ranked, but in theory you can learn them at any order. In practise, it's exceedingly rare that any higher level abilities are learned before gaining a good foundation by mastering a few low-level abilities.
- The times I mentioned that one usually has to spend are per ability, not per level. If one wanted to master all four lvl 5 abilities, it'd take around 10 years, not 2 and half. And that's intensive training, unless you're an exceptional genius.
- Learning an ability and mastering one are two very different things. The times I mentioned are for mastering abilities, but they're also for focused traning. You can learn to use an ability in half the time (of intensive training), but your grasp of the ability will be shaky: the ability will be weak and unfocused compared to if you'd mastered it, and it can fail you at a critical moment. Or you can master an ability spending twice the time in years, but less time each day, leaving you plenty of time for other things.
Understand that ki is not easy. Compared to magic, it's difficult, in addition to being draining on the body. It's a poor replacement for natural magical abilities - or god-given magic - it's not even meant to be equal to magic as far as game balance is concerned. But, it can be learned by anybody, even those not lucky enough to be born with magic, without having to serve a god in exchange. It's a power entirely your own, not borrowed from the nature or a god. For many ki-users, it's a thing of great pride.
Edit: One more thing I just realized I forgot to mention: Ki-abilities that work within the body aren't terribly consuming, they mainly just stress the body, instead of consuming ki. Abilities that expel ki from the body are much more consuming, but they generally aren't as stressful to the body. Both tire you out, but in a different way. Losing ki is the more dangerous one and usually recovers slower. (Setsuna is an exception to this rule as well, his ki recovers much faster than that of a normal human.)