I have a Nissan Primera P12 2500cc sedan (2001) at around 97,500 km on the odometer.
I asked my mechanic when do I have to replace the timing belt and he said it lasts the lifetime of the car and doesn't need replacement.
Although everywhere I look, many people say it needs replacement otherwise it will break damaging the engine. I would like to know whether I need to replace it and when?
I replaced the serpentine belt. I assume this is not the same?
The engine in your Primera should be the QR25DE. According to the Wiki page, it should be equipped with a steel timing chain, not a belt. Your mechanic is spot on. Timing chains are usually good for the life of the engine (rare occasions otherwise).
A timing belt/chain connects the camshaft and crankshaft together, making them work as one. The camshaft rotates at half the speed of the crankshaft, but timing is crucial between the two (therefore why it's called a timing belt/chain). They are used on nearly all overhead cam designed engines. With the proper covers in place on the engine, you won't see the belt/chain at all. You are correct in thinking the serpentine belt is not the same as the timing belt/chain, but are wise in keeping up maintenance on it. They do wear out!
EDIT:
There's no easy way to tell without dismantling part of the engine. You may be able to take the front top cover off of the engine to see (this should be an aluminum cover which will be on the front side of the engine - to the right side of the car as you're sitting in the driver's seat) . Everything I've been reading show the P12 with the 2.5l engine to have the QR25DE, which is chain driven. I don't think you'll have anything to worry about.
There are really two related questions here:
How is the engine constructed (timing belt vs. timing chain), and
What is the service interval on the belt or chain.
As has been pointed out there are design tradeoffs. Chains, in general, tend to last longer. Belts, in general, are quieter and might be easier and/or less expensive to change.
The service interval determination involves weighing a number of competing considerations. For example:
While chains are more reliable the belts, they do wear and can fail. Regular oil and filter changes (since the chain is usually (always?) running in the engines oil bath) will help extend the life of a chain.
A chain the wears out and fails beyond the useful economic life of the vehicle is a "lifetime" chain. If you have a classic car or figure the useful life of the vehicle differently then you may need to plan for replacing evan a "lifetime" chain.
The consequence of a chain or belt failure. Some engines, called interference engines are designed so that the cam and crankshafts must be in time. If they are not (e.g., if the belt or chain breaks while the engine is running) then it is almost certain that the valves and pistons will collide doing significant damage to the engine. Diesels and other high compression designs are almost certainly interference engines. It used to be that interference engines tended to use chains, but belts have become more reliable and more common as service items and that rule doesn't hold as well as it once seemed to.
Wear on a timing chain will cause it to elongate, and over time this may affect the performance of the engine as it will shift the valve timing slightly.
Verifying that you engine has a chain can be done by identifying the engine and then checking its description. On engines with timing chains the chain is internal to the engine and may be hard to inspect without significant disassembly. Engines with timing belts will usually be designed to facilitate inspecting the belt as it is a wear item that has to be replaced. The timing drive will be on one end of the engine or the other and if it is a belt there will likely be a relatively easy to remove cover that lets you inspect it.
