Insulation that has been installed properly in the attic or anywhere else can last from 80 to 100 years.
However I have had to change out some insulation that has sunken down in an attic before but I believe it wasn't installed properly.
Make sure you install the correct R Value of insulation in your attic for your area but if you buy the Insulation Locally from hardware stores in your area then it will be the correct R Value.
Insulation in the attic can help keep the heat inside your home and prevent it from escaping through your attic which will help keep the house warmer and lower your heating bills.
It can also help keep the house cooler in the summer months and lower your air conditioning costs as well.