Level 1 skills should take ~10 minutes.
Level 2 skills should take ~30 minutes.
Level 3 skills should take ~4 hours.
Level 4 skills should take ~5 days.
Level 5 skills should take ~20 days.
I'm not sure how long it takes to learn every single skill, but when the most expensive skill only takes ~5 days to complete, I feel like I'm going to run out of skills to learn very soon.
When I played EVE Online, there were multiple high-level skills that took upwards of 30-45 days to complete, depending on your skillset (Better Learning V, etc).
Thoughts?