Sometimes during surgery the nerves may get damaged and it can lead to numbness for a long period of time.Actually no one can tell that how long it will take for the nerves to get normal but it takes on an average about 6  to 12 months for numbness to go and get back the normal sensation.But in some it may take about 2 to 3 yrs as well. You can visit a dentist and get a routine check up done for ur satisfaction............for more info refer the link..........http://www.identalhub.com/dental-complications-after-tooth-extraction-881.aspx