This is an inexpensive part and if it is making any unusual noises it is past time to replace it. It takes more time to talk about the part on the 'net than it does to just change the thing. I'm not sure what the manual calls out for on a replacement interval, but somewhere in the every 50-70k mile range is probably about right. Loose the belt and you have no alternator, no power steering and no A/C. Why mess around?

Unfortunately Volvo put the water pump drive on the timing belt, which is pretty brain dead IMO. This way if the water pump happens to seize up then you grenade the whole engine when valves and pistons come into contact. Most makers drive the water pump off the accessory belt where a water pump failure is more benign. Just ask the 850 and 960 owners who have had this nightmare happen to them :(.

If you are not worried he's lying, you have one answer to your two questions. When the belt makes an airy whispy sound, it's time to replace it. I think others will give you the actual number - should be every 60-70k. Keep your records. Do you know when it was last changed? By the way, when the belt breaks, you will know how bad the problem is.

I've seen any number of broken serp belts lying on the road. Look at the belt for cracks, missing ribs, etc. They can last indefinitely but will be destroyed by a mechanical incident such as a seized power steering pump, etc., and are badly affected by air pollution such as ozone, esp in hot climates. Sounds can come from any of the rotating components, such as alternator bearings, p.s. pump, a/c clutch, serp belt idler, etc. If the belt seems ok, when the sound gets loud enough, you will know what the source is.

I had the serpentine belt on my Corvette break on Christmas Eve while on a long trip. Not a pleasant experience and it leaves you stranded until it is fixed. I replace ours at about 50K miles. They also make a lot of noise as they get older.
--
1998 Volvo S70

my volvo dealer says to change belt every 70,000 miles. i have heard that some the of the newer volvos = 100,000 miles. the problem is that if the belt breaks you can toast your engine. as madeinjapan suggests, inspect your belt.
i had the dealer change mine at 70,000 and 140,000 miles.

NMI

Moregolf said he thought it was a 30K mile thing and he changes it every 2 years. I think that's a waste of money and energy. Actually, Volvo's maitenance schedule calls for this belt every 60K miles, but most don't do it then...they wait until the timing belt at 70K miles. Remember that it also pulls your water pump and power steering fluid. You can easily look at it without removing anything to see its condition. If it has small cracks in it and you don't know when it was changed last, I'd say you should do it. With every other Serp. belt change, you should change its tensioner.
--
1998 S70 T5 Emarald Green Metallic (125K), 2004 V70 2.5T Ruby Red (45K), Previous Owner of Black '94 850 Turbo Wagon (Sold at 140K and miss her). My cars have been running so well lately they've got me worried.

I change mine every two years. I think it's a 30K mile thing. Pretty cheap maint.