I'm currently downloading GTA V from Steam to my SSD and it will take up about 80GB of space.
My SSD's capacity is 240GB, but after the download it will be left with a small amount of free space of about 30GB.
Does this reduce its lifespan ?
No, any 'normal' use won't strain a SSD into an early grave, however, having a low amount of free space can cause SSDs to run slower.
It seems the general rule is to only fill a SSD to 85-90%.