"long-time user" is correct and usable in written English.
You can use it when you are referring to someone who has used something for a long period of time. For example: "The software engineer has been a long-time user of the application."
Mrs. Gordon, an 85-year-old resident of Silver Spring, Md., was a satisfied, long-time user of the federal government's old Legacy Treasury Direct system, which allowed users to buy Treasury bills and other securities by mail or phone and receive paper statements.
Like many other music services, iTunes offered users the ability to purchase individual songs just as unauthorized services did, thus satisfying the long-time user desire to purchase parts of collections rather than bundled selections as copyright owners had been loathe to abandon.
Celebrities on Instagram is nothing new — Snoop Dogg is a long-time user, for example.
Nor is the agency a one-product shop; it's also a long-time user of IBM's SPSS tools.
As a long-time user of Airport devices I'd definitely miss Apple's fire-and-forget interface.
Although a long-time user of social media, IS activities took a significant turn in September 2015 when the group's official media outlets took to Telegram.
That decision left many long-time users irate when Auction Assistant stopped working on their computers a few weeks ago: not subscribing meant not receiving necessary updates.
For long-time users of Twitter, this type of downtime is nothing new.
That being said — the conversation view is bound to cause some complaints among long-time users.
Despite new features targeted to companies, Todoist For Business' user interface will look familiar to long-time users.
It's actually very common for modern-day optics systems, but long-time users aren't really used to it.
