For PBS without commercial interruption the PBS Redbook shows a runtime of 26:46, of which the last 6 secs contains 1 sec black and 5 sec "This is PBS". For PBS Plus or APTS the timing isn't as strict. Commercial shows TRT is 28:30 or 28:50 with actual show time at 22:00. In today's market it mainly depends on what the presenter (Discover or History) generally uses, which should be in their show specs.
fire*, smoke*, photoshopCS3
Charlotte Public Television
You really need to have the delivery specs. For instance, Discovery wants an international "snap-in" which is an extra few minutes of footage that they can cut into the show to make it longer for int. release.
It sounds like you don't have a sale yet and you want to cut to time. That can't really be done, so I would suggest you cut for 28:30 keeping in mind where you could easily lift out up to 6.5 minutes worth of material. Then deal with it when you have a sale.