may I know how much do you usually put?

It's more about getting the right keywords or phrases, making sure that your ad and website is relevant to your keywords, and what you need to bid in relation to your end profit margin. For instance, if your affiliate commission on a product is say $20 and you get on average one conversion per 100 clicks to your advertisement, then if your cost is $0.20 per click you only break even. If your cost is $0.15 per click you make a profit of $5 per sale. That may not seem much but it's a 33% return in your investment. You won't get that sort of return from any bank. It's a numbers game.

Mine is already 2 days.

I would check back with them

Around 6 keywords. The delivery status is "eligible'.
How long it takes for your ad to be approved?

Okay, I started out with about 20 keywords and then over a couple of weeks, trashed the low traffic ones. Also, as Kyle suggests, I started out with a higher budget than I really wanted to .... $20/day and then lowered it later. The PPC companies (Google, Bing, Yahoo) will pay "more attention" to a higher set daily budget (initially) and gear more traffic to these campaigns. Just monitor it closely (three or four times per day) and you'll be fine. But, I would suggest increasing the number of keywords also. Start broad and then after you begin seeing traffic, trash some of the more obscure keywords and you'll discover the ones that are really working for you.

I just checked and seems that some of my keywords falls into the category of "good quality but low traffic keywords". Maybe that's why it takes longer? Will try to make some adjustment and see how.

Yep, that happened to me on a few.