As long as I can remember. Anytime I'd see a new doctor they'd say "You have a heart murmur." I'd respond with "yep, I know." That was all that was said. I've had arrhythmias my whole life too; that's a different story but still, they seem to be benign as well. I can't imagine having a quiet, "normal" heart. Kind of scary :-)
So you had this your whole live I assume? Always regarded as innocent?
That was interesting. I've heard of innocent murmurs but never flow murmurs. I googled the term and the first site had the sound of a flow murmur available. That's just how my heart sounds but I call it my whoosh/thump. Doctors say I have a murmur but it's never been a problem and I'm 56 now. Guess they're right. :-)
I've had a flow murmur for my whole life (33 years) and never could get a real reason on why I have it. Echo only showed increased flow through my pulmonary valve and slightly increased flow through my aortic. The cardio said I have a normal heart but I still didn't believe it... I thought flow murmurs were supposed to go away after your teens.
So in the same situation; tests are normal but still wanted answers. I think these things are best to just shrug off. Maybe others with innocent murmurs will chime in and share their stories as well.