What is the most efficient way to do periodic checks in C#?

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C#

I think constantly checking DateTime.Now against an end time that updates after the end time is reached seems computationally intensive, and I was wondering if there is more computationally efficient way to check a condition periodically.

What I have tried:

DateTime CurrentTime = DateTime.Now;
DateTime EndTime = CurrentTime.AddMinutes(1);

while (TRUE)
{
CurrentTime = DateTime.Now;

if (CurrentTime>EndTime)
{
EndTime=CurrentTime.AddMinutes(1);
DOSOMETHING;
}
}