My response will be in part influenced by the information I gathered from reading your profile.
My first suggestion to you is to strongly encourage you to learn an instrument. If you're serious about writing music and about having it played by live performers, having a working knowledge of the instruments is important. It is paramount to be technically proficient on at least 1 instrument in order to understand how sound is produced and general performance characteristics.
Before I answer your question, I want to clarify that the term diminuendo also known as decrescendo means to gradually decrease in volume, not to "hold until you run out of air." In considering the answer to your question, there are many variables to consider:
1.) The proficiency of the performer (professional / college / high school / etc. different levels will be able to provide different amounts of breath support.)
2.) The range you are asking the performer to play in.
3.) The tempo of the music.
4.) The dynamic of the pitch in question.
5.) The time signature being used.
6.) The context of the note (whether it is the held note at the beginning / end of a phrase or if it is stand-alone, etc.)
7.) Whether or not the performer can circular breathe.
As you can see, it is impossible to give a simple answer to this question, as the length is contingent upon the variables I outlined above. Sibelius will be able to hold the note indefinitely because it is a computer and therefore does not need to breathe.
Having a dynamic of "ppp" is different than having the performer physically play until they run out of air - the composer will find different sounds for each of these.
I believe your real question is something to the effect of "what is the clearest way to notate a performer holding a note until they run out of breath?" To which my simple answer would be: it depends on the context of the piece.
That said, in a general sense, a clear way to notate that would be to have a fermata over the note in question with a diminuendo ending not in a dynamic, but with "n" which stands for "nothing." You could also put a note above the staff saying "fade until nothing" in which case the performer would automatically adjust as necessary.
Hope that helps.