All it takes to ruin a tip, is to drop the iron on its tip *ONCE* from a height of a foot or so, or to press too hard when soldering. The tip bends slightly cracking the iron plating, letting solder through to dissolve the copper underneath.
You can also ruin a tip by letting PVC or other plastic containing halogens burn on it. Acids from the burning plastic rapidly erode the iron plating exposing the copper core.
Also any sort of acid flux (usually ones containing zinc chloride) can ruin a tip in a few hours use.
Cheap tips often have thinner plating than the genuine OEM ones for your type of iron, and tend to wear out quicker. High bit temperatures can destroy a tip quickly - anything over 400 deg C (~ 750 deg F) is likely to reduce the usable life to days or even hours.
Also if you use a wet sponge for tip cleaning, if its too wet, it can thermally shock the bit and possibly crack the iron plating, or mineral residue from your tap water can build up and attack the plating - always wash out the sponge thoroughly and squeeze till nearly dry before use, and if your water is heavily mineralized, use deionised water to rinse the sponge after washing it, and to wet it as necessary during the day.
The first time you use a new tip its important to get the whole working surface properly tinned without delay, up to the line where the plating surface finish changes. Keep the temperature as low as possible - but solder should melt readily if rubbed on the bit - untill its fully tinned.