It is harder to correlate the grade of a heart murmur with the severity of the underlying heart problem in cats than it is in dogs.
Some quite loud murmurs may occur with relatively small defects.
It is sensible to assess the patient for clinical signs that would suggest an underlying problem (e.g. lethargy, abnormal breathing pattern or effort, pale gums).
The presence of such signs indicate further diagnostic work up such as echocardiography.
However, if the cat appears very well, is showing no other clinical signs of a problem, and exercises normally, then it is fair to suggest a repeat examination in a few months to reassess the heart murmur and see if it has changed, or to see if the cat has developed any other clinical signs.