I installed film on all the windows in our previous townhouse about 10 years ago. I think the brand name was Llumar, it cost about $300 and was enough for ten windows without much left over.
There was a window in our master bedroom closet, and after living there for about 6 months, our clothes, curtains, and furniture were fading, even though we kept the blinds drawn the majority of the time. After installing the film and living there another 8 years, I didn't notice any more fading.
My TZ sat with one of those filmed windows in pretty much that same location while we lived there, but I kept blinds and a blackout curtain on the window all the time. Plus, it already had some fade on that side.
Oh, and you can get pretty much the same treatment pre-installed on your windows, but I am not aware of any aftermarket product other than the film. It is a PITA to install, and if I had it to do again, I would have professionals do it.
They just installed it on the windows in the office a few months ago, and those windows are about 6 feet wide. I don't know how they handled pieces that big.