I’ve been trying to work out how long today’s current loose and very natural style of long hair, plus the later side parting, has been around ?

And I keep thinking it must really have started back in the early nineties or so with Farah Fawcett Majors in some sleuth (?) TV series with two other actresses…? Forget what it was called now but it does make this style a very long running one indeed, especially as I can’t remember what the popular style was before that  !  :)

I think you're thinking of Charlie's Angels... I'm not sure about the hair question though.

I, too, think you are talking about Charlie's Angels, only about a decade earlier than that, I think.
Don't know about the hairstyle, but you could be right. Although F.F.M's hair was, I think, what we call "big hair". Lots of volume. Later on the styles were more close to the head.
But I am no fashion expert!

"Big hair" usually refers to hair with lots of volume, typically achieved by ratting the hair.  FFN's hair in Charlies' Angles was called a "shag".  She had naturally wavy/curly hair that was cut with many layers to emphasize the curls.  It was a very "revolutionary" style that worked well with long and medium length hair that had a lot of body and waves.  

As for it being the beginning of today's longer, natural hair, I'm not so sure.  I think that some hairstyles are cyclical.  After a period of very short or very complex hairstyles, I think a rebelious faction of women decide to quit with the torture and let their hair go natural.  Or maybe it is just another way the fashion industry tries to control people - look - here's something else you can't have! (Like the pencil thin legs, pouty lips, no hips or rear, etc, etc, etc).

I think that hair industry may have something to do with keeping the natural trend up. When most people can do only a ponytail or leave the hair down, all the styling has to be done by a professional.
On the other hand, if updos suddenly became popular and updo instructions were easily available, people might be less interested to change their color and cut as often as they now do.
Besides, the "natural" down styles we now see are seldom really natural: people use lots of product and heat tools to make the hair fall just right and achieve the desired texture.

So, I think this trend may be around for a long time.

I used to watch Charlie's Angels so I can remember that it was the mid 70's when the series was pretty popular.  The other stars besides Farrah Fawcett were Kate Jackson and Jacyln Smith.

Kate Jackson had shoulder length hair and Jacyln Smith had BSL hair and she was my favorite Angel 8-)

Her hair style did influence many young ladies and girls but so did Dorothy Hamill's very short hair (if I remember right that was the mid 70's about the same time as Charlie's Angels?).

Jerry

Dorothy Hamill's cut was more like the late 70's, early 80's .  I'll admit that one of my drastic cuts was from waist length to a short bob like Dorothy's.  That was in 1981 (and I was 8 months pregnant :o).