I have a rogue class who was lvl. 13, who is currently a lvl. 12 ninja. I do not want him to go through any class changes any more, except to become an assassin. When should I do this? I am just going into the stout mines and am wondering if I should complete the assassin's quest while I am there, or wait till my ninja is a higher level... Any suggestions?

Personally, I choose to change classes as soon as I have all my current skills at a good level. This way, I don't have to worry about losing skills at a class change (for example, not learning any Sun magic as a Wizard and then changing into a Warlock--you can't train or practice Sun magic now).

So, my advice would be to gain as many levels as you need to max your favorite skills (or get close), then ascend to Assassin. But, if you change him early, you can gain the HP bonuses sooner, so it's really just up to personal preference. =)

Could someone give me even a lvl.!? not specifically, but maybe a range or an estimate?

Anyone?

As far as I know Assassin is a super-set of Ninja: I think they can do/learn everything a Ninja can and a bit more. So no reason to wait with changing really.
The only thing you might consider is to make another character Assassin: if you e.g. switch a paladin to assassin (OK, not the best role-playing choice perhaps) he gains more new skills/traits than if you switch a ninja.

The assassin is an "invite only" class. In order to become an assassin, you need to have the dagger that you get from the bandit (I forget his name right now) that you meet at the abandoned house north of Ishad N'ha. He asks you to go kill a guy in the stout mines. When you do that, he gives you the dagger. Take that to the pawn shop.
good luck.

Note that even when you've done the assassin's quest and received the dagger, you can still decide not to visit the rogue's guild with the character with the dagger as selected char. But the role change is offered only once, so once a character is offered the change, and you decline, it won't be asked again to that character.

And of course if you haven't installed the patch, you can create as many assassins as you want whenever you want.