I'd say it needs around 500 hours playing time to reach max level. Maybe less if you are an experienced player. Usually experienced players manage to get from level 1 to max level in around a month. Inexperienced players need around 3 months.
Good things about Aion:
- great graphics and nice scenery
- high quality ("polish")
- good PVE and PVP (quite PVP-driven, though, as pure PVE player you will have problems)
- fast-paced combat with many skills
- thousands of quests, no need to grind as there are always more quests available than you can even do without reaching max level, some quests are actually quite good, others are just the normal "Kill X mobs", story is fairly good as well and very character-centered (usually MMORPGs have more world-centered quests and the character is just one of many "adventurers")
- possibility to grind is still there, they made the perfect balance between questing and grinding, so no matter what you do, you level up at about the same speed
Aion is THE recommended game if:
- you are looking for an F2P MMORPG of high quality and good PVP
Aion is NOT recommend if:
- you want action-based combat
- you are a pure PVE player
- you want something really unique
Only real alternative to Aion is Allods if we go by quality and F2P.