There is no specific rule of thumb.But, one should have have blood pressure every 2 years by the primary care doctor if it is below 120/80 and yearly if blood pressure ranges from 120-139/80-89 according to the 2007 United States Preventive Services Task Force (USPSTF)guidelines. 
In the setting of blood pressure >120/80 with severe headaches,vision difficulty,dizziness,loss of consciousness,chest pain,leg and arm weakness,nausea,vomiting and fatigue one should seek for urgent medical care.Hypertension is generally associated with a wide variety of symptoms but generally causes problems to the heart,brain,eyes/vision,kidneys,nerves and arm/leg weakness.
When to seek urgent medical care when I have Hypertension ?
There is no specific rule of thumb.But, one should have have blood pressure every 2 years by the primary care doctor if it is below 120/80 and yearly if blood pressure ranges from 120-139/80-89 according to the 2007 {{ORGANIZATION}} (USPSTF)guidelines. 
In the setting of blood pressure >120/80 with severe headaches,vision difficulty,dizziness,loss of consciousness,chest pain,leg and arm weakness,nausea,vomiting and fatigue one should seek for urgent medical care.Hypertension is generally associated with a wide variety of symptoms but generally causes problems to the heart,brain,eyes/vision,kidneys,nerves and arm/leg weakness.