Hi RG! It really depends on your skills and strengths. My suggestion to you: take a sample IELTS practice test and then you'll get an idea what your language proficiency level is.
I took GRE 5 years back and tried IELTS general. It was pretty easy. Not too tough for someone who took TOEFL. once you take sample, you can make out which section you need to concentrate more. 3 weeks is something which again depends on individuals.
I am a graduate and all my studies were in English medium. I do have 2yrs work experience in US and about 10yrs in IT.
I found IELTS quiet easier than I thought. I had prepared for over a period of 4 weeks - 1hr a day and 2hrs over weekend. I scored 8 band ( L=8, R=9, W=7, S=7).
If you can prepare a schedule and work more on the areas where you are weak, I think it should be possible in 2 weeks.