Different schools have different requirements for test scores. You will need to look at the websites of the schools to which you plan to apply to see what they say about the scores that they want to see.
Depending on the program, you may need to take the GMAT or the GRE. Some doctoral programs in business require the GMAT, but the GRE is the more standard test for doctoral programs in general.
Most people spend about 3 to 4 months preparing for either the GMAT or the GRE. If it has been an extra-long time since you were in school, then you may need more time to get back up to speed. (Most people take the GMAT and go back to business school within about 5 to 7 years of finishing their undergraduate studies.)
Once you've done the research to figure out what test to take and what kind of goal score you need, if you need to take the GMAT, start here:
https://www.manhattangmat.com/blog/inde ... our-score/