I downloaded Daggerfall a week or two ago, and so far I have been playing on this Khajiit Rogue.

But, I was thinking about starting over, since everyone hates me and a couple other reasons.

Anyways, my question is:

Would a Khajiit Nightblade be useful in any way?
And how long does it take to get the invitation to the Thieves Guild, because I've picked like 11 pockets, and the Wiki says you need to pick 10.
It's been about 3 days?

Nightblade should be fine, though it might take a bit of time for one to really be effective in combat (weapon skill is only major, and Illusion's a bit of a weak magic school in Daggerfall, particularly in combat). Learning the custom class creation system might get you a bit further, but it's optional as long as you're careful, and you'll die a lot no matter what you do.

I think three days is the average or the minimum amount of time to receive the Thieves' Guild message, so you're still in decent shape. Feel free to go around picking a few more pockets while you're waiting just to make sure you got enough. Also, make sure to check your packs often - the game often forgets to tell you that you've received a message (it'd be quite fitting for you to suddenly realise you have a letter from thieves without realising how it got in your pack, though. <_< ).