"have long noticed" is correct and usable in written English.
You can use it to indicate that you have been experiencing, witnessing, or observing something for a long period of time. For example: "I have long noticed an increased level of noise from the neighbors' house."
Anthropologists, of course, have long noticed the importance of tattooing.
Although researchers have long noticed that the brain is inflamed in Alzheimer's patients, he had dismissed inflammation as a major factor in the disease.
— Researchers who study the beaches of the Outer Banks have long noticed that storms cause particularly severe erosion in areas they call hot spots.
But doctors and nurses treating the mentally ill in Britain have long noticed that a very high proportion of their patients are serious users of the drug, often starting to take it at a young age.
I am bemused because I have long noticed that almost all economic pundits and soothsayers — whether on television, in newspapers, or at brokerage firms and conferences, are asked to tell the future.
The book is full of insight into the complicated mechanics of Tiger Woods's golf swing, and offers some wonky explanations for things that fans have long noticed: why Woods cannot drive the ball straight (fear of the driver), why he struggles in the opening rounds of major tournaments (plays too conservatively), and why, in recent years, his putting has gotten less reliable (not enough practice).
Baker said he had long noticed the racial disparity between first- and third-base coaches.
Mr. Burch's half-brother Frederick Caldwell has long noticed that Mr. Burch feels most comfortable in interracial settings.
Like other blacks, Ruben Burch had long noticed that blacks had never been cast as Joseph, Mary or Jesus.
Ocular melanoma specialists had long noticed that some patients did well and the rest did not, but Dr. Harbour wanted to know why.
He was amazed and pleased to learn about the cavern, having long noticed a hollow sound as he rode his horse over the site.
