Hmm, that probably depends on what you define as "benign". You could define benign as "false positive", like you would frequently get in women who are still menstruating. You could also call asymptomatic stones "benign" hematuria as well. Benign could also be all include all of those cases in which the hematuria reamins idiopathic (like in people who have drug reactions, diabetes, bph etc). Or are you calling benign hematuria all hematuria that does not come from malignant bladder cancer? Anyways, just to give you some statistics, according to Current Medical Diagnosis and Treatment, in patients with gross or microscopic hematuria an upper tract (kidneys and ureters) can be identified in 10% of cases. Upper tract causes include stones, renal diseases, renal cell cancer, and transitional cancer. Microscopic hematuria in males is usually due to benign prostatic hyperplasia, and I think that the most common cause of "microscopic hematuria" in menstruating women is a false result from their menses. I have no idea how many people in the general population would have microscopic hematuria at one time. Asymptomatic gross hematuria is usually due to bladder cancer.
