There seems to be a lot of confusion regarding IR timing/duration. Let me try to clarify it. The following discussion is not specific to the Philips pronto but rather to IR patterns in general, i.e. what the IR emitter is doing. I will discuss it in terms of Pronto HEX format as that is a good format for describing timing.

In Pronto HEX (leraned) and in many other similar formats there is a series of numbers that represent on/off times. The on time specifies how long the IR emitter is to flash at the IR carrier frequency, and the off time how long it is to be dark emitting no IR. In Pronto HEX this is called a "Burst Pair"

The last Burst pair consisting of one on time and one off time, is what is normally referred to when adjusting the duration.

The last off timing item shuts off the IR signal for its duration and then, well it does not turn it back on, so IT STAYS OFF.

What it really does is to tie up the hardware for that duration of time so another signal can not start. This is important in a macro where you may have two successive commands addressed to the same device. The device must be allowed time to process a command before the next one starts. It is generally irrelevant if the two successive commands are addressed to different devices. The fact that the IR emitter is off is what is important, how it got that way is not.

The last off timing is also relevent when a command is repeated as some protocols use a sequence where the command is sent once and then a special sequence is sent which says repeat the last command. The final burst timing on the repeat command might be quite different then the final butrst timing in the command itself.