Different schools have different rules about this. For some schools, you pre-req courses will never expire. For others, you have five years. Some schools are six or eight, perhaps, but I think five is most common. So, if you took chem I in 2005, you have until 2010 to get in. If you go past the five year mark, you must retake any pre-reqs that expire.
There are a couple of ways around this if it's only a few courses (like, I think yo can be exempt from retaking some things if you work in a related field, or recently took some hardcore upper division classes)...
Does that make any sense at all? Lol.