CAT

CAT and XAT are very similar on some notes but very different on other notes. Similarity:
Quant and Verbal Section
The quant and verbal section of both CAT and XAT have the same chapters and more or less, test the candidate on similar lines. In the verbal section also, both the exams are filled with RCs which check the candidates reading and analytical skills. Difference :
Difficulty & Number of Questions
CAT will have a total of 100 questions visa vis 72 questions of XAT. Since, the number of questions are less within the same time frame, XAT. Difficulty level goes up. Replacing LR-DI with DM
The biggest difference. XAT has a section called Decision Making, where they give you a small case and ask questions based on that case. Its a typical case analysis with options (that's where it ges subjective). CAT has LR-DI section which have ballpoint answers. An Extra Section
XAT has GK + Essay Writing which CAT doesn't have. While this section isn't important for the first shortlisting criteria but may be important during the final conversion. Negative Marking
CAT has 1/3rd negative marking against 1/4th of XAT. But XAT has negative marking for unattempted questions too. If you leave more than 13 questions, you'll be awarded 1/20th negative mark for each question then.

Undoubtedly, CAT is a tough nut to crack. How much time is required for the preparation to crack CAT depends upon the dedication and smart preparation by the student. CAT has three sections. Quantitative Aptitude, Verbal, Data Interpretation and logical Reasoning. Number of questions in each section:
Quantitative Ability: 34
Verbal Ability & Reading Comprehension: 34
Data Interpretation & Logical Reasoning: 32
Total: 100
Now, actually, most of the syllabus of CAT comprises of what we already studied in class 9th and 10th. And to pass through CAT with flying colors, one requires a good understanding of Syllabus, Time management technique and Smart answering technique. If one prepares for it with dedication then 6-8 months would enough for cracking CAT and getting a call from top most institutes for an average student. I hope that I answered your query well. Good luck.