How much does it take to diagnose an arrhythmia?

Capturing it. As a clinician you can strongly suspect it from a patient's description of symptoms but to accurately diagnose it you need to be able to record a symptomatic episode with some form of ECG (there are many modalities available that allow recording episodes even remotely).
EKG. Like any diagnosis, the history and physical provides clues, then, ideally, the arrhythmia would be present on a 12-lead EKG; however, the way an arrhythmia starts & stops can be recorded on a heart rhythm monitor. Some monitors are worn for days to months. Implantable monitors look for years. An invasive electrophysiologic (EP) study can provide critical details. See heart rhythm specialist.
Arrhythmia. there are several methods from ECG to Holter and even sometimes implantable devices to diagnose an arrhythmia ...