Part of the SSD stays "free" anyway. It is called over provisioning and all modern SSDs do it afaik between 10-30%.
Thus even if you use your SSD 99% for weeks it shouldn't have any problem as it technically isn't full yet but still has some extra space that you cannot use up.
There is no need to keep 30% free. It is simply really annoying if you always run out of space and there are quite a few programms that simply crash if the hdd tells them it is full, thus you should always strife to keep around 10% free so you don't run into such troubles.