I took the NCLEX on yesterday, so I will find out tomorrow if I passed, but I should be able to give you some advice. I don't think you can ever do too many practice questions. I studied for a solid 5-6 weeks and tried so do a couple hundred questions a day, at least, as well as reading review material (I primarily used Davis's book, Kaplan Strategies book, Saunders, and the Delegation, prioritizing, and assignment book.)
You have to be prepared to sit for all 265 questions, so I would suggest you do as many questions in one sitting as you can, so you build up your mental endurance. As far as burnout goes, just make sure you finish your studying 1-2 days in advance of the exam. Give your brain a rest and do something fun/relaxing. Your brain will thank you. The day before my exam, I spent the whole day cleaning the house, which took my mind off the test. I went into test day with a clear mind and a clear head so I could focus, and I think it worked
I took 75 questions...and I'll find out tomorrow!