The phrase "a long time partner" is correct and usable in written English.
It can be used to describe someone with whom you have had a long-standing relationship, whether in business or personal contexts.
Example: "After years of collaboration, we consider our client to be a long time partner in our success."
Alternatives: "a longtime associate" or "an enduring partner".
A consummate gentleman, Bill was a long time partner, colleague, friend and inspiration.
Sports brand Nike, a long time partner of Apple since the days of the Nike + iPod, will have one of the first fitness apps available for the Apple Watch connecting to the Nike Fuel system as well as Apple's Heath app.
El Mini Lic is the son of Dámaso López Sr., a long time partner of El Chapo who is alleged to have been instrumental in his first escape from maximum-security prison in 2001 — this came 13 years before the infamous drug lord was first recaptured and held for 17 months before he escaped again in July 2015.
A nationally-recognized energy regulatory attorney, John was a long-time partner at Chadbourne & Parke LLP.
He was a long-time partner of Seymour Schneidman & Associates and Anchin, Block & Anchin LLP.
"Egypt is a pivotal country in the Middle East and a long-time partner of the United States.
LONDON – General Electric agreed on Friday to buy the Italian aerospace company Avio for $4.3 billion, acquiring a long-time partner in its jet engine business.
Mr. Gruson, a long-time partner with Shearman & Sterling, was a well-known and highly respected attorney practicing in the United States and Germany.
Georgie Harman, the chief execution of beyondblue, a long-time partner with Norton, said the mental health organisation's work was increasingly being carried out digitally.
Disney, a long-time partner of Pixar whom Mr Jobs broke with when he got tired of its former boss, is now trying to worm its way back into his favour.In short, Mr Jobs currently seems vivacious by anybody's standards.
"He was already softened" after his public humbling in 1985, says Bruce Chizen, the boss of Adobe Systems, a software company that is a long-time partner of Apple's.
