I can't go into specifics of the actual exam questions or assignment subject, as this is contravenes the test taker agreement that we all sign up to at the beginning of the exam. Outside of that I'll do my best to answer.
Q. What not to study for multiple choice exam. There are topics like salesforce mobile, apex chart, visual workflow these are not specified in study guide so can I ignore these topics?
A. I don't remember any questions that were not based on GA features. I did make sure that I was familiar with the information on the beta/pilot features that were available, but mainly from reading the release notes and developers guide.
Q. From start to end, ie appearing for 1st exam to receiving the result whats the total duration it took for you?
A. Just over 6 months, but some of that was waiting. I took the first exam around the middle of May, but then had to wait until September for a chance to sit the assignment. There's around 1 month to complete the assignment, and then a 6-7 week wait for the results.
Q. If somebody fails in programming assignment than he/she has appear again for multiple choice again?
A. I'm not 100% sure, but I believe that you can retake the programming assignment without having to retake the multiple-choice exam. This is based on comments from someone on LinkedIn though, so it would be better to check this with the certification team from Salesforce
Q. The 20 hrs of timeline is actually restricted to 20 hrs by monitoring login time or you can take more time and login to sandbox/production anytime for your assignment?
A. No, it was made clear that 20 hours was the suggested minimum amount of time it would take you to complete the assignment, there was no mention of any upper limit. I recorded around 28 hours, although some of that was researching and building the QA scripts.
Q. Any tips apart from the blog post?
A. The multiple choice exam is a little different from others I've taken - the questions tended to be a lot longer (including code snippets in the main), so there was more to understand and take in. More knowledge of reference material was required (e.g. triggers and order of execution, standard component attributes). Aside from that, the usual tips of ruling out any obviously wrong answers to narrow down the opportunities, making sure that you read the questions and answers properly.
Q. When is the next registration window for the programming assignment?
A. According to Nina Marinova on Linked In, the assignment window opens Dec 21. Full details at: bit.ly/sXmySO
Update - the registration for the assignment was full in a few hours this time. All the more reason to ensure you are following salesforce certification on linked in, facebook and twitter.