How long meth stays in your system, and whether or not it will show up in a drugs test depends on a few variables, it is not an exact science.
The first thing that can effect it is a persons physical make up, namely their age and weight, and the speed of their metabolism. There are also various other unknown factors which influence the length of time taken to break down a chemical which will vary from person to person.
The other thing to consider is the type of drug test being conducted. Some are more thorough than others, and test for different things therefore the results may vary. In urine, blood and saliva tests it is estimated that meth will be able to be detected for between 1 and 4 days, where as with a hair follicle test, it may be evident for up to 3 months.