The phrase 'have long realized' is correct and usable in written English.
You can use it when speaking about something that you were already aware of for some time. For example, "I have long realized that a good night's sleep is essential for maintaining my mental wellbeing."
Philosophers and psychologists alike have long realized that thinking is not of a "single piece".
As they struggle to understand modern climate change, scientists have long realized the importance of reconstructing past climate variability.
Doctors have long realized that regular exercise could prevent and control high blood pressure, diabetes and heart disease.
However, vested interests have long realized this and engaged in a campaign to misinform the public about the scientific consensus.
Public health officials around the world have long realized that condom distribution is central to any meaningful AIDS-prevention effort.
Of course, many wealthy families have long realized that their children needed to learn about money whether they wanted to or not.
A few people had long realized how much she was working.
"We've long realized we're at a relative competitive disadvantage when it comes to higher education," said Bradford L. Smith, Microsoft's general counsel.
The Jewish settlement movement, added Mr. Ezrahi, has long realized that in normal times the consensus in Israel does not support its calls to build more and more settlements or to expel Arabs.
'Iran and Hezbollah are an inseparable part of the Syrian atrocities and the world needs to act against them,' the Prime Minister said". Israel has long realized that the devil the world knows in Damascus is worse than most anything that would follow his removal.
