The most widely recognized Japanese language exam in the world is the Japanese Language Proficiency Test (JLPT). Its primary objective is to measure and certify the proficiency of non-native Japanese speakers.
There are five levels of the JLPT, with N5 being the easiest and N1 being the hardest. Note that the difficulty progression becomes steeper as you move through the levels. As an example, you could take six months to attain N5 level coming from a zero background, but it might take you a few years to get from N2 to N1.