Safe LED Operating Current
It is the luminous intensity of LEDs that decreases over the life of a LED. Higher the current you supply to LED, higher will be its power dissipation, and its temperature will increase. This induces thermal stress and electrical stress on LED and speeds up the rate of decrease of luminous intensity. What you are asking for is the derating of forward current for improving reliability of LED.

Answering your first question, if you supply 20mA forward current then the luminous intensity will degrade by the percentage shown in below graph.

You should determine if this percentage of degradation is acceptable to you. If not, you have to reduce forward current. 

You also need to consider effect of temperature on maximum forward current. 20mA maximum current is at 25 degree C ambient temperature (mentioned in your datasheet). But you also need to derate (reduce) this with increase in temperature.

Please see this datasheet form Kingbright 

PS: I have used it only as example. The ratings are different from your LED.



This curve shows the derating you need to consider as effect of ambient temperature. So consider the maximum ambient temperature in which your LED will operate and then determine the maximum forward current at that temperature.

And in doing all this, the result you get is decreased luminous intensity, which is shown in graphs as below.



To conclude, in your case the power dissipation is too less (~150mW). So just limiting the current to 15mA will be fine if you are satisfied with the intensity.