As I recall oblivion will not feature the ability to join the imperial legion, so I plan to make a mod to fix that sad situation though this will depend on if I get a computer to run it and if they have any specific reason for not including the imperial legion (I never go against the official lore). Obviously because the game is not released yet I have nothing solid but basically I was thinking about 3-4 quests to gain a rank but about 8-16 possible quests to do per rank, but complex tasks may count as more than one quest. The type of quests you do will depend on your rank of which I expect there will be about 8-10 of but in the main groups of militia (fighting type quests), guard (solving crimes to hunting down criminals) and champion.
At a certain point you will have the option being adopted into the order of a fort as a knight, you can only join one order and they will have a similar set up as guilds in Morrowind except it is for high level characters and it will have a larger impact on the game. Orders will include one which is your typical questing knight, a battle mage order, a stealthy archer order, an imperial cult order and 2 others which I am keeping secret. I will be looking to recruit people but not yet this is just to let people know what I am planning to do if anyone’s interested.