Actually, as you progress through higher levels of skills, the time involved in learning them increases. This is taken from the wiki:

Tier 1 - .5 days
Tier 2 - 1 day
Tier 3 - 2 days
Tier 4 - 3.5 days
Tier 5 - 6 days
Tier 6 - 8 days

All upgrades to each skill takes 1 day, though Tier 1 skills upgrade in .5 days.