Good tips to look for when wanting to know when to re-grip your clubs.
Signs of when you should replace your grips are when your grips become shinny and difficult to hold on to or your grips have started to crack or peal, and in some extreme cases your grips can change shape with wear and leave thumb or finger wear marks. You should ideally replace your grips once a year of course this will depend on how often you play and where your clubs are stored.
I have seen players come into me with grips only a few months old and have worn them completely out although this is rare it can happen especially if you have a poor golf hold. I have also seen the opposite with players getting 4-5 years from grips again this is rare and only with synthetic grips the rubber grips will harden for sure after 18 months even if you don't play golf.
As rubber naturally disintegrates over time when changes occur in its molecular structure. These changes can be accelerated by exposure to ultraviolet light (as in sunlight), heat or cold. (Ask your professional about the wedge he forgot about in the pro shop window). Both natural and artificial rubber are susceptible to chemical changes and the changes are, unfortunately, irreversible. Once these changes take place its time to change your grips.