What's the normal lifespan of an alternator? (Gen 3)

I've ordered my 2 replacement belts from eBay, current plan is to try swap them in at the weekend!

@Lambert

If it's more of a chirping or a screaming noise it could be the idler pulley bearing. 

It's both a chirping and a screaming noise Basically if it's a colder / wetter day and I start the car and sit still - especially with high draw electrical components on like heated mirrors & headlights - then it'll quietly chirp-chirp-chirp-chirp-chirp. When I then put it into gear and start driving it screeches like a banshee for maybe the first 30 seconds of the journey. My usual response is to let my foot off the accelerator and roll a little (turn off heated mirrors if on) which makes the noise go away. Sometimes after doing that it immediately fixes it for the rest of the journey, other times I have to do a few "cycles" of screech-and-roll before it goes away.

What never happens is screeching after a long drive or randomly 5 minutes into a drive. It's exclusively to do with start-ups, presumably because the alternator works harder to recharge the battery.

Oh another thing... I've seen "uprated" alternators for sale on eBay (e.g. tinyurl.com/ebay-alternator-jimny ) that claim to output 100A as opposed to the standard 80A. Is this tosh or do they actually work well as an upgrade? Saw some other threads where people put in as much as 150A alternators from other vehicles but that required some more advanced modifications than a straight swap.

If you're getting belt squeal with a standard alternator, a bigger one will make it worse.
I remember seeing an alternator upgrade someone did on a Jimny a while back where they fitted an idler wheel to increase the belt wrap. Even then it's pretty marginal to run a 100+ amp alternator on a small 4pk belt. Most larger alternators come with a 6pk pulley but to fit a 6pk belt you would have to change all of the other pulleys, which is a pretty major job.

As I recall, my gen 3 used to screech a bit on start up, so I tightened the belts a bit, then after a while the screech and a vibration happened whilst accelerating too, then one day all the noise went away, along with the outer half of the crank pulley and the belts.....

@LesNewell - That's a really good point I hadn't really considered! I guess there's only so much juice you can get out of a 4PK belt with no idle pulley to imrpove grip etc.

@facade - That's pretty disturbing about the crank pulley disintegrating!! What was the vibration like? I have found that I sometimes get a low rumbling vibration when sitting in Drive but with my foot on the brake (worse in Reverse). Once I start moving it goes away. I thought it would be the "transmission / gearbox mount" but even after replacing that it's still there. I get a slight rumble when I'm idling and use the window switches too lol but I think that's just the alternator being worked.

The vibration was only slight, but I could feel it through the pedal.
Mine was manual, and it happened when pulling away just before it was time to change up.

The pulley is incredibly poorly designed, tensioning the alternator belt tries to rip the pulley apart, and is resisted by tensioning the power steering belt.
Grab the outer flange of the pulley and try and rip it off the car, if there is any movement whatsoever, it is time for a new one.

Here is my sketch of a section through the pulley to show what I mean.