I made some experiments for testing various leds
some old ones from old stock that I got ( > 20 years) (OS) (Old Stock)
some other new from cheap chinese source (N) (New)
I have a high voltage DC source.
I made a small device to be able to test several leds at once.
They are all fed with the same DC source.
Then each is in series with a red led.
If the breakdown occurs, the red led light. I then remove the tested led.
A voltmeter and ampmeter are in the circuit.
The results for the first batch are
10 V : 5mm green OS
23V : 5mm red OS
25V : 5 mm red OS
39V : 5mm Yellow OS
46V : 5mm red N
47V : 5mm red N
48V : 3mm red N
53V : 3mm red N
57V : 5mm blue N
68V : 3mm green N
73V : 3mm green N
80V : 3mm yellow N
84V : 5mm blue N
96V : 5mm green N
99V : 5mm green N
113V : 5mm yellow OS
Then I used some other leds to make a more systematic scan fro the same batch of leds
A) red 3mm (N)
47V ; 51V; 53V; 53V ; 54 V
so quite consistent, and from the previous test can also be added (merging 3mm and 5 mm)
46V; 47V; 48V; 53V;
B ) green 3mm (N)
81V ; 91V; 91V; 101V; 101V ;
and from first exp ,
68V; 73V; 96V; 99V
C) blue 3mm (N)
27V, 28V, 28V, 47V, 62V
and 57V, 84V from first exp.
D) red 5mm (OS)
21V; 21V; 28V; 30V; 43V;
and from first exp :
23V; 25V;
And in summary
red (N) mean 50.2 V std 3.2V
green (N) mean 89V std 12.3V
blue (N) mean 50.8V std 21.7V
red (OS) mean 27.3V std 7.7V
So the green are definitely more robust than the red. But the old red are less robust than the new ones;
Of course, this may well depend on the kind of leds.
I have randomly check that all the leds are still in working condition.
I may test later on how high they can survive on AC voltage. But this will be destructive .