new growth only. the reason people like to stretch so much is because there is a lot less overlapping of the chemical on the hair when you relax so much at one time. If you keep re-relaxing your hair strands, they will get weaker until they break and they won't be healthy.
Some people even put vaseline, condish or EVOO on already relaxed hair to preven the relaxer runoff from re-relaxing the hair in any way.