There’s no hard and fast rule on when you should change your tyres but generally speaking so should look for wear indicators on road/hybrid tyres. These are small circular indents are around 2mm in diameter and as they get shallower you should think about renewing your tyres. Ripped and torn tyres should be replaced immediately because they render the tyre dangerous to ride in most instances.
Ironically if you haven’t ridden your bike for a while it may be time to change your tyres. Even if your tyres have tread left on them and don’t have and tears or splits they may have cracked.
Simply put rubber deteriorates over time. A bike that has been left in the shed, garage or worse of all outdoors for a prolonged period of time may have cracked tyres. The varied temperature range, moisture, UV exposure and many other factors will break down the rubber. When you come back to your trusty old steed and give the tyres a blast of air don’t be surprised if you see small cracks and micro tears on the sidewalls parallel to the rim.
To prevent your tyres from cracking you should keep them out of direct sunlight and store your bike clean and dry alternatively don’t store it and keep on riding.