For bushings, if you're in dusty/water-y conditions alot, it could be weekly.
To kill the squeak, NOLATHANE bushings. They're a urethane bushing, but are much more flexible. Also they have a checking on the inner bore to hold grease better.
For a full leaf refresh you want to get the Spring Eye bushing and Leaf Spring bushing kit
Improved ride, handling, and load hauling capability for your 2014 Nissan Xterra are just a phone call away. We have the performance suspension parts you need.
www.carid.com
or from their direct web page PN# REV161.0054
REV161.0054 Spring kit - All in one leaf spring bushing service kit covering front and rear eyes of the leaf plus shackle bushings. High kilometres and heavy load carrying causes excessive wear on rubber bushings. Nolathane bushings are made tough and offer outstanding abrasion resistance, high...
www.nolathane.com.au
You can also find them on Amazon, SummitRacing and others, just verify the PN# from Nolathane's webpage for the individual bushing 'kits' (spring eye front, eye rear, shackle, LCA/UCA, shocks...)