ip nat translation port-timeout -- WHICH port?

Opinion:

I suspect that it would influence any translation referencing the port number (inside global, inside local, outside local, outside global).

Consider HTTP translations rather than NTP, because the client and server ports would differ with HTTP.

Compare a translation for an outbound connection to an Internet-based Web server, and another translation for an inbound connection to a web server within your organization.

In one instance it is the outside local and outside global addresses listening on port 80, and for the other it is the inside global and inside local addresses listening on port 80.

Presumably, you would expect/want both translations to be influenced by your NAT translation port-timeout configuration. If not, you'd have to settle for a generic TCP timeout, or configure a timeout for all possible client ports for one of the two scenarios.

Once connection initiation triggers the translation, I would expect traffic in either direction would reset the timer.

Best Regards, News Reader