If palpatation is caused by something serious, such as ischemia, how long , on average, does the issue cause problems if left untreated?

Palpitation is not a common symptom of ischemic heart disease but it may. Palpitation is more common in people with irregular heart rhythm. The risk of untreated cardiac arrhythmia depends on the type and cause. You should see a doctor for a thorough history and physical exam to define the nature & cause of the palpitation. However, palpitation is common & not always associated with heart disease.