Someone's idea is an eternal entity that is on hold until its procedure to be creat or form an idealism or materialism.
Once the idea has materialised then it has become reality thus this eternity has ceased.
Eternity is a concept not rather then a physical truth and can only be measured against time/reality.
Eternity is wheel of time that once reaches spefics/details it stops to project and reproduct.
In other word eternity is never lasting because of reality.