In my experience burning out alternators on a Pontiac and an Oldsmobile, you can get about 100,000 miles out of it before expecting to replace the alternator.
form_title= Automotive Alternators form_header= Ensure you car will run with an alternator. How old is your current alternator?*= _  How long have you owned your vehicle?*= _  When was the last time you had you car in for a tune-up?*= _ 
In most cases an alternator will last well over 100,000 miles
It depends on just how 'bad' the alternator is, but generally, not very long. You can actually drive your car until there is not enough electrical charge to spark across the spark plugs, and your car just stops, unable to start again.
Depending on how long the interval was between alternator changes. Less than a year and I would look for warranty on the alternator. Now there are factors beyond the alternator manufacturers control. A battery that is on it's last legs will take out an alternator really fast. A stereo system with a large Amp(s) will also shorten the life of an alternator drastically. These situations will not be a warranty issue.
It's not a good idea to to run your alternator with bad diodes. These diodes rectify the ac power generated by your alternator and turn it into dc power. You will destroy other electrical components in your car and will end up spending more money than the cost of simply replacing your alternator.
Impossible to answer as it depends on use and abuse. Normal use will see no need to replace one on a modern car.
I have a 2003 Yukon with 211,000 miles, the only thing I have reoplaced is the battery and the alternator.
Yes, as long as the engine is running, so is the alternator. The alternator is actually what keeps the car battery charged.
Spark Plugs do not affect your alternator. As long as your engine is running then it should be charging unless your alternator is bad
Time. Auto batteries share one thing in common with humans - no matter how long they go on, they all die eventually. they last longer if well maintained, and with a good alternator, but they all die eventually.
i have 163,000 on my 97 Sebring and it is still running strong except the alternator just went out. Then it will be a champ again!!!
On most cars built within the last 10 years the voltage regulator is built into the alternator.