I use the Head Hydrosorb Pro synthetic base grip. Always use Head Prime Tour Overgrip with it and it lasts for a long time, 12+ months, however, removing OG eventually wears it down especially at the buttcap area where overgrip sticks to it -- eventually starts ripping off. That is when I change them.
I no longer use leather grips but back in the day I would replace them. Today, I wish I hadn't. Leather can roughly last forever as long as you clean it from time to time and possibly condition it too. I would not change a leather grip if I were to use one again.