I have some left over MP miles that I was hoping to use for a business class upgrade but am short by 600 miles. I noticed that with the Hertz 500 mile online booking promo and double miles I can get them for much cheaper than buying 1000 miles. However I'm not sure how much time does it take for the miles to be deposited after I return the car. Hertz online chat cust rep told me usually it takes a couple of days, while on the phone they told me 6-8 weeks! How much time does it really take?

Are there any other cheap ways to get these 600 miles quickly (say within a week or so)?