How common is long qt syndrome and heart arrhythmia problems in a young female?

Uncommon. Long qt syndrome occurs in approximately 1 in 10, 000 people. It is more common in females, by an almost 2:1 ratio.
Quite rare. It is believed to be genetic in origin and quite rare, but can be devastating as it can be the origin of sudden cardiac death from ventricular tachycardia or ventricular fibrillation. It can be exacerbated by several different types of medications of different classes, the most common of which are psychotropic medications such as benzodiazepenes, lithium, antipsychotics and some antidepressants.