Is it normal to experience post op paresthesia (numbness) for an extensive time after dental treatment?

Possibly: Paresthesia may occur for a variety of reasons. If the cause of this condition is infection than it is abnormal and needs attention. If the cause was traumatic it may take an extensive time for the condition to reverse itself. Unfortunately when caused by trauma it does not always reverse itself however this is uncommon and usually predicted prior to the procedure. See your doctor for more info.

What kind of dental : After a wisdom tooth extraction that has its roots near the nerve yes. After routine dental proceduressuch as filling, deep cleaning, crowns.....It is not normal. Unless lots of anesthetic was placed and the needle has hit the nerve while numbing you.

No: If i read your question carefully, you are asking if it is normal after a dental treatment (say surgery) to have paresthesia? And if so how long? It is unusual for it to occur at all, but if does i would say it would be normal for the paresthesia to take 6 weeks to several months.