How Long to get #1

We will try to get on #1 on a 500k keyword.... #1 Amazon #2 Nextag #3 Target...

We are on 4 page now with 1.5 year old domain

How long will this take

What about backlinks for the ones already ranking ? PR?

If you are on page 4 after 1.5 years, (imho) you're not going to make it.
I took a VERY HARD keyword and placed it 1st/2nd and it took me 4 months.
That was the fastest i could do it mostly whitehat.

Im not sure what you are into but you might start looking at getting aggressive.
But only push it as far as you need and then maintain.
good news.... you're already on page 4! at a year and a half old domain, give it hell

You cant be #1 in all the 500K!
cuz each keyword nead to work hard for it to be #1, and first check out who is in the #1 now in all those KW and see if you can beat them all or not?
I think its better to take them 10 kw each time and focus on the ranking if you do it take the other 10 and so on ...