Sorry if this is a stupid question :P

This is not stupid at all. It actually depends on how much of your aerobic capacity you use, your ability to process and break down lactic acid build up coupled and the time it takes. Also important is your age. As a guideline, somewhere after around 40 seconds is where one enters the phase of aerobic glycosis. This assumes one is working at near capacity level. Take an 8 year old that can swim 50 meters in 57 seconds.....while a 16 year old can swim it in 30 or slightly more. The 8 year old has broken the aerobic barrier whereas the 16 year old has not. Also when training, if this is done as a set and repeated, the rest period comes into play as a short rest period leaves one repeating the distance at a deficit. It actually gets even more involved than this but this is not something to be explained here. There is alot of physiology consideration and human performance issues backed by scientific data to explain it. Many people get masters and doctorates in just these issues and scientific courses of study. Our Olympic coaches generally have this kind of schooling background as a minimum to even be considered for this job.