Re: stock tyres and durability
There are markings (mostly little arrow heads) on the tyres near the thread edges as a guide to the wear limits. It is recommended that if the tyres' surface are close to it, it's about time to change.
Alternatively, if you study the marking on the side wall, there is usually a 4-digit numbers embossed on the tyres e.g. 2804. In this case, it means the tyres were manufacture on the 28 weeks of the year 2004. For time expiry, I would change my tyres if it has pass 2 years old as the rubber compound would have degraded, causing noise and loss of grip.
Other time to change would be tyre damage. It can be cause by bad road surface, bad or poor suspension setup, brakes and at times, uneven wear may occur due to camber and toe. This is usually seldom notice unless you inspect your tyres. For suspension related damages, you need to feel the tyres and see if there are lumps or slight undulations. For this, you need to change your offending suspension as well.
Puncture would be obvious and usually, it can be plugged. Change the tyres if you have more than 2 plugs. If puncture occur on the sidewall, forget about plugging it, usually doesn't last and can be dangerous at speed.
Hope the information helps,. at least to make the sotong little bit smaller.
P.S. If you drives hard, or one of those that do the Sepang thingy once a while, you cannot be a tyre sotong coz it is VERY IMPORTANT.