Replacing the lower pulley is easy as you just remove the bolt and slide out the old, slide in the new and replace and snug the bolt.
Pulleys on off-road used MTBs or on road bikes ridden in gritty and wet conditions and/or with a poorly maintained chain can wear fairly fast. If they are kept clean and dry, a life of 20,000 miles or more is quite possible. I have a 105 9-speed rd with 25,000 miles and the pulleys, while a bit worn, are still in usable condition and the bike still shifts well.