How can I determine if a Brocade SFP is nearing the end of its lifespan?

Over time the output power and input sensitivity of an SFP (Tx) deteriorates. You can measure "how much light" is received on an SFP by using web tools and looking on the Tx power:

Also by looking at the Rx power you can see whether an SFP receives enough light to operate without problems.

But my question is: what are the upper and lower limit at which a Brocade SFP operates?

In a Cisco switch you can see a warning level and an error level, but I don't see that in Webtools (Brocade).