"well established since" is a correct and usable phrase in written English.
You can use it to indicate that something has been established or accepted for a significant amount of time. For example, "The policy has been well established since the creation of the company."
These practices have become well established; since the 1970s, the Public Affairs Council, the association of public affairs officers from many of the world's largest corporations from Dow to Disney, has held an annual National Grassroots Conference.
While methodologies for the integration of heat have been well established since the 1970s, the integration of heat and work is much less discussed.
In France, where the Senegalese community has been well established since the 1970s, hometown associations exert a strong social pressure on migrants to encourage them to keep financial connections with origin families and communities (Chort, Gubert, & Senne, 2012).
Antiestrogen treatment has been well established since the 1990s.
The involvement of ethylene as a regulator of root growth has been well established since the early experiments of Smith and Russell (1969) with roots of barley.
"A national dependence on accessibility to EpiPens has been well established since Mylan's acquisition of the device in 2007.
It's true that there have been some technical innovations, but the mechanics of passive solar design were well established 30 years ago.
It is well established today that heterologous overexpression of proteins is connected with different stress reactions.
The concept of multilayers technology to improve the performances of existing polymers is well established today.
The benefits of oral appliance therapy on upper airway dimension in OSA patients are well established
While medical guidelines are well established earlier, the standardisation is achieved by defining time boundaries for each step in the process.
