How long does irritability associated with teething last?

Depends. All babies are different, but on average a baby starts to teeth at approximately 6 months with the central incisors through 30 months for the second baby molars.
No set age. There is no set age for when it stops. Teething usually begins at 3-6 months and can last until 3 years of age.
Individualized. The symptoms of teething will last as long as your child is teething. Teething rings and damp frozen wash clothes for you child to chew will help the teeth come in sooner. Use the appropriate amount tylenol (acetaminophen) for severe teething pain. Avoid using medications that you rub on the gums b/c it only will last for 10 to 15 minutes and will not provide lasting relief.
When tooth erupts. As teeth erupt babies can be fussy, sleepless and irritable, lose their appetite or drool more than usual. Try rubbing their gums with a clean finger, a small cool spoon or a moist guaze pad. A clean teething ring or pacifier will help. Be patient.