Will add a reference to the discussion in Part 2 that explains the factors that affect the choice of value.

Will not define lower and upper bounds, only add discussion about the factors that affect the choice.

Look up text in Part 2, add reference to Part 2 and check if we need some text from there. Check with Part 2 updates.

Added the following paragraph to the parameter description in OPC UA Part 4 - Services RC 1.04.14 Specification.docx:

Several cryptanalytic attacks become easier as more material encrypted with a specific key is available. By limiting the amount of data processed using a particular key, those attacks are made more difficult. Therefore the number of exchanged messages between Client and Server must be limited by establishing a new SecurityToken after the lifetime.
The setting of the requested lifetime depends on the expected number of exchanged messages in the lifetime. A higher number of messages requires a shorter lifetime.

Agreed to changes edited in Raleigh meeting.