Low powered LEDs in clock radios last over 10 years, which I suspect is where the 100,000 hour rating comes from. High power LEDs due to thermal and other issues do not last as long. IIRC posts by CPFers working on LED lifespan show 5000 to 10,000 hours at normal drive currents. That is still over ONE HUNDRED battery changes.
The life is much reduced when the LED is overdriven voltage and/or current wise or when the LED gets too hot.
Coin cell keychain lights (2x2016) overdrive the LEDs but are no problem as the batteries give out before the LED.
Cheap 3xAAA LED flashlights on ebay or from far east vendors are a problem. They are usually direct drive. Alkalines will give 4.5V and NiMH or NiCd will give unlimited current at over 3.6 volts hot off the charger. This is more than the 3 to 3.6 volt 20 mA reting of most 5mm LEDs.
The exception to the rule are the 3xAAA 3 watt Luxeon or clone. These LEDs can take 700 mA which an AAA alkaline battery will be hard pressed to produce for any length of time. No garuantees though.
Cheap also means shoddy construction. Bad clicky switches, cold solder joints, springs that are not long enough, painted over or corroded contact surfaces and a total lack of quality control may mean a non functioning light even if the LEDs are good.
Brighter is not necessary better if you are worried about LED life. A while back flashlightreviews reviewed two 4 AA 7 LED lights - The Streamlight propolymer 4AA 7LED measured 21 total output (x1.43=~30 lumens) and the Nightbuster Aqualed measured 14 (x1.43=~20 lumens). The rating of the Streamlight was later dropped due to reports of burned out LEDs.
Be careful of heat dissapation requirements when modding or upgrading an incan. flashlight. Terralux warned the maxstar should only be used with the maglight (metal body). There was no warning with the Quadstar but I managed to burn out both of mine the 1st evening in PLASTIC flashlights. The 4 LEDs in the Quadstar produced as much light as the 7 LED Aqualed (way overdriven). And they were HOT when I transferred them between lights. I did not have problems with other admittedly dimmer bulbs from Sino Union, LedCorp, Tectite and SuperbrightLEDs.
In general LED lights will last at least 20x as long as an incan light with the original bulb. Buy only brand names from a retailer with a good replacement policy just in case you get a dud.
And stay away from lights that are trying to be the brightest in it's class. Currently there is a fierce competition over whether the Peak Marathon Ultra or the Arc AAA-P is brighter. Both companies are driving the LEDs to the design limits. If I want the 'best AAA light' I would probably get a NON ULTRA 3 LED marathon instead. The LEDs will probably last longer and 3 LEDs will give me the same amout of light. As I am not too fussy or rich I ended up with a Dorcy AAA in my pocket (US$6 at Walmart USA or C$13 at Sears Canada) and a UK 2AAA eLED on my keychain (US$12. at pocketlights.com and diving shops)
I would suggest the Dorcy 1AAA 1LED as a good starter LED light. It is relatively inexpensive. Small and pocketable so you have it with you always to try out in different situations. Can be mouthed comfortably leaving both hands free.
In a month or 2 you can make up a list of wishes for your definitive LED flashlight - brightness tint runtime size weight and go to town at flashlight reviews, LED museum, runtime plots by Roy and the CPF reviews forum.