Sorry just checked mine and it is about 10 years. But that will also depend on how you use it.Unless you plan on constantly moving large amounts of media, your drive shouldn't begin slowing down, until after the life of your computer is over (the other components will age too).
One disadvantage of putting OS on SSD is that it can get cluttered quite quickly - lots of articles in PCA hard copy about this. Possible to overcome but it looks as though one must be aware of the problem. Apart from that the advantages are great. If and when I get a new toy I will get an SSD for the OS.
About ransfering large amounts of data or more specifically "writing".
IF I had for example pictures on my HDD and I used Windows Picture Viewer to view them, am I correct in saying that the pictures wouldn't be written onto the SSD, but onto the RAM where the Picture Viewr would also be? Same question for videos and music. And so viewing media written on HDD wouldn't affect the number of writes left?
Similarly, if I was watching a Youtube video from a browser, this would be downloaded to RAM?
Furthermore, I am worried about running programs from SSD and how they use up all the writes?