How long will a 1W LED last without a heat sink

presume you checked the data sheet for correct volts and also looked on Dr Google

Heat is the #1 led killer then overvolts/transient spikes

FWIW

LED lights for domestic use, what a minefield.  My main use ED 15W lamps from Wilko (12hr/day 365 - 9000hrs) lasted ca 2 years - note these do get hand v hot

Same from Ebay 1/4 price lasted 3 months - ca 1000hrs

I discovered that mains LED lamps with their enclosed electronics dont like being switched on/off frequently so use in a PIR lamp in a pub toilet failed in a few months - would have been cheaper to leave it on 24/7

I found for example that a porch light activated by a plug in adapter for low light detection was very poor because these cheap plugs have fluttering switch levels near dawn/dusk which quickly killed the led lamp

It was cheaper to leave the lamp on 24/7 - to date 8 years - 70,000hrs

So beware of cheap led from PRC, its invariably factory rejects ending out the back door onto the black market

but then thats a whole can of worms (eg trying to source genuine Eneloop AA NiMH batteries, so many fakes, some even recycled and repacked with <50% capacity viz Dr Google

Counterfeits ( Lion batteries) are such a major headache for Panasonic, Samsung etc (even tho they get their goods made in China).  Imagine your EV full of fake batteries OMG

@robint Thank you.  I bought the LEDs from an eBay seller, from China, that came in bubble wrap without any documentation at all.  I just assumed I'd never find a data sheet.  The purchase was a while ago too.  Perhaps I'll just go find a random data sheet and see what it says.  

@tperry724

IMHO go for a look alike with data sheet and derate volts by 10-20% for longer life. i found prc sellers grossly over state their products (which were factory rejects anyway). Check the heat with one of those IR hand held temp guns - good stuff.  They tell you if you  are Covid hot to trot

@robint        and tperry724

Both you and tperry724 leave me very puzzled about what you are talking about, probably because "LED light" is not well defined.  Are we talking about LED the little pea sized transparent or translucent plastic indicator light with two wires sticking out or about LED the replacement for an incandescent light bulb? I've used both with few problems.  The light bulb replacement fails occasionally (more than I think it should) but I'm suspicious the problem is in the built-in electronics, not the LED itself.  

Regarding the indicator light, the 3.7v you mention is the forward voltage required to just get a bit of forward current and it's the current which gives the light and heat, the more current the more light until it burns out. The normal way to use these is to use a power supply of 5, 6, even 12 volts and a series resistor to control the current. And the LED is non linear so ohms law does not directly work. So the rough and ready way to size the resistor is to subtract the forward voltage of the LED from the supply voltage and use the resultant voltage and 85% or 90% of the rated current to calculate the resistance by ohms law.  The resistor wattage would then be current squared times resistance.

There is a neat graphical way to get a more accurate estimate of the resistance but this requires a VI curve of the LED (and I haven't discovered how to put a drawing on this forum). 

Yes indeed quite right.  It always pays to go the extra mile to understand how something really works, rather than a quick fix.  Some try to get by without Ohms law in this digital world

As for posting pix, this is a secret known to the Grand Wizard and his coterie not made obvious to neophytes but I will impart the good oil

you should see at the bottom of the reply box (I didnt see this at first)

@robint

OK I think I've got the "attach file" working so I'll whip up an example of the graphic for all to admire

Here's an example of a graphical method for selecting the series resistor for an LED. The same general approach works for many (or most) non linear devices.

you wanted to post a pic?

****************************

Illustration of a graphical method for sizing the series resistor for an

LED

 This data was taken from Shenzhen Fady Technology Co. data sheets for a 5mm            LED. Other sizes are similar but not identical. 

Basic Method:

1/ Get the LED VI curve from the manufacturer's data sheet    and sketch it on graph paper. Super accuracy is not needed here except     for reasonable accuracy at the current you intend to use.

2/ Mark the power supply voltage V on the voltage (horizontal) axis.

3/ Mark the point on the LED VI curve at the current you intend to use.

4/ Draw a line from the power supply voltage mark V, through the    current mark on the VI curve to I on the current (vertical) axis.

5/ The series resistance needed is then R = V/I   Watts = Imax2*R

@robint

All right,, you're up on me.  The picture you've posted of my attached file is what I wanted to do in the first place but could not figure out how.  So how did you do it??

attach jpg, copy txt

Thanks,  I guess the problem is that I attached a .pdf, not .jpg