Short answer - 500 is pretty simple to score
Now, this is a subjective question because some people may find that they need longer time to achieve 500 as compared to others.
You should start browsing some information on the test. Take 2-3 days, get to know the test. Then, give a practise test. you will get it free when you register for the test at mba.com
If you score somewhere like 450 then you will be able to get your 500 in 2-3 weeks time. If you score more than 500, then I think you don't need to prepare much .
Prep courses are not required. buy the official guide for GMAT 12 edition, start solving the questions.
morning or afternoon- depends on you. my mind works best at 11 am....so i chose 10 am as a time.