The length of time it takes to prepare for the GED tests vary from one person to the next. I have had some students take as little as 3 months to complete taking all 5 tests. These are students who have a pretty high reading level. Others, whose reading/math/writing levels are low may take as long as a year or more. Another factor is how much time a person can devote to studying. The more you study, the quicker you will complete.
Having good GED books is also a huge benefit! Purchase one book which contains all 5 GED areas, or purchase individual books for GED math, GED writing, GED science, GED reading, and GED social studies.
Check with your state to see if you can take one test at a time (my recommendation) or if you must take all 5 tests at once.