Completely depends on school and course! I initially targeted and applied to 5 different schools and they were all different. If there was a time limit, most were 10 years and they were the hard science courses. But some schools put that limit on more courses (had to retake English comp) and other schools had no time limit at all. One of my schools was able to waive some of the time lines. First go through every page on their admission site. Then check the ptcas program directory. Finally, call them and ask very specific questions. You don't want to be caught on the wrong side of this one.