I think reaching a new level should be Hard, as in I DO HOPE AND PRAY that it's impossible to reach max level in a week. Even if you play with a life support machine by your side!
Leveling should take its big chunk of time, half of your level should be gained trough quests/pvp/wtv and rest by grinding mobs.
In the furthest levels, assuming i'm playing 4 hours per day, it should take me 1 week of more to level up, i dont expect levels too be offered like in some MMORPGS currently in the market. It must be earned
(however no need to exaggerate)