As long as you want it too...
It is all down to regular servicing. Most modern engines that are less than 15 years old will go on and on as long as the important wear and tear items a taken care of.
A Busso V6 engine is extremely strong, the 2.5ltr being the strongest simply because it has less stress to handle than the 3.0 or 3.2 variants.
Be fastidious with servicing and your engine will thank you for it...
Every 6,000 or 6 month an oil and filter change 10w40 semi synthetic will suffice but 10w60 fully synthetic will help if the V6 uses more than 1 litre per 2000 miles through burn off, it will reduce it considerably. 10W 60 is a lot more expensive but if you find that the engine is indeed burning of oil naturally which many Alfa's tend to do, then it becomes cost effective to prevent it from doing so.
Every 12,000 or 12 month air filter change paying attention to the air box lid seal and make sure the air box is cleaned of debris before a new filter is fitted. This will prevent MAF issues. Clean the throttle body and inspect the rubber induction hose for splits, no matter how tiny they can effect the running of the engine and lead to other failures in sensors trying to balance out irregular air flow.
Every 24,000 gearbox oil, power steering fluid and brake fluid needs renewing
Every 36,000 new spark plugs (all six) they last a long, long time but are pricey because of that fact.... Make sure that the rear bank was actually changed last service.... Many places and owners don't actually bother due to the extra hassle of removing the plenum chamber. Make a point of inspecting the rubber boots on the separate coil packs for damage, splits or even chuncks missing, this will lead eventually to misfires if left unchecked.
Personally, I also drain and refresh the engine coolant at 36,000. I also remove and replace the thermostat at the same period as a matter of course and give the radiator a thorough inspection with the bumper and fan cowlings removed... It may seem that I like to waste money, but nothing could be further from the truth... Just 3 month of running around in a V6 with a slightly faulty thermostat will use more fuel than what a new thermostat actually costs!
Every 60,000 The big one... Cam belt and auxiliary belt change along with both tensioners and all idlers... If the water pump was also not done, change that too for a 3.0 metal impeller type... Then simply replace it every other cam belt change...
I can't emphasize enough how important this cam belt routine is... It is really the only weak link in the Busso V6 and if kept on top of, there is no reason whatsoever that your engine wont last at least 350,000 mile without a complete rebuild at one stage in its life...
Mine is quite happy at 112,000 miles and my father-in-law has just clocked up 210,000 on his V6... both are just run in
Then there are ancillary parts like gearboxes, clutches, alternators and starter motors
Gearbox will last as well as the engine will. The differential being the weak link but can be upgraded considerably with a Q2 diff or similar performance variant.
Clutches can last between 60,000 and 100,000 dependent on the owner and driving style... They are a struggle to change though... Take the opportunity to change the starter motor at the same time as that is also a nightmare to remove by itself and a lot easier to do at the same time.... I would also recommend getting the alternator checked and/or repaired if not charging well enough when doing the cam belt..
Doing all these things reduces the running cost with forward planning considerably. Before you know it, you have a car that is cheaper to run in maintenance and servicing costs than a 1.9 or a 2.4 JTD.... What you loose in fuel economy, insurance and road tax in comparison, you can gain back by keeping the servicing right