The pH test is designed to measure your natural ability. So typically 3-5 days of brushing up of concepts should suffice. Most of the surveyed candidates,who have cracked companies which do challenging and innovative work or pay in excess of Rs. 6 lakhs per annum, took the eLitmus pH test within 2 weeks of getting to know about eLitmus / registering on the eLitmus website.
All required formulae are provided in the question paper. You can refer to any standard aptitude material to brush up / revise your concepts for the eLitmus pH test.
Please remember that natural ability cannot be improved overnight by mugging up questions from 4-5 books. That approach may work for other pre-employment tests or for MBA entrance tests like CAT, but it is not helpful for the pH Test. This is what makes the test unique and consistently helps companies get superior work force which in turn helps them win against their competitors (majority of eLitmus' customers grow faster than their competition).
Also there is a misconception that the eLitmus pH Test is difficult. Most of the candidates who make it to jobs with rigorous selection process report near zero preparation. This is because the test is conceptual, and candidates with weak concepts find it difficult and ones with strong concepts need no specific preparation. For example, in quantitative sections, 12 out of 20 questions can be solved in less than a minute by candidates with strong concepts. Last but not the least, scores are given in percentile terms. So it does not matter even if the paper is difficult, as it will be equally difficult for all test-takers.