The phrase "a long reminder" is correct and usable in written English.
It can be used when referring to a reminder that is extensive in length or detail.
Example: "I received a long reminder about the upcoming deadlines and the necessary preparations for the project."
Alternatives: "an extensive reminder" or "a detailed reminder".
I tried to stay positive even though I knew what was to come: a 45 minute long reminder of how far I am from my fitness goal.
Fortunately, it was fixed shortly afterward, just a long-standing reminder of Murphy's law.
A long-overdue reminder of what the Wii is all about.
This experiment in training and appointing should hopefully provide a long-term reminder of the problems of sharp implementation of new ideas without proper consultation.
Given the disruption to the body — the traditional approach requires splitting the breastbone, stopping the heart, and using a pump to oxygenate and circulate blood — it's amazing that most people come through bypass surgery with only some scars as a long-term reminder.
Certainly, the fear of violence over Ayodhya was only the latest in a long line of reminders that, as the columnist Vir Sanghvi put it, "as hard as we try to build a new India... old India still has the power to humiliate and embarrass us".
His office glowed at the end of a lightless road, a large whitewashed building with low outer walls and a long driveway — an eerie reminder in this desolate province of the British-colonial pretensions (and origins) of Pakistan's omnipresent bureaucracy.
Ross Barkley will face more testing opponents than Estonia but his performance at Wembley last Friday, including one of the best through-balls I have seen for a long time, was another reminder of his rare qualities.
And his performance, at the end of a long album tour, was a reminder that new work can take time to settle in.
