iPlayer Radio: Under The Hood
The quick answer is "they're different per station, they're different whether live or on-demand, and they'll change at least another two times this year". If that satisfies you, you have no requirement to read on. If you want more information, however, I'm happy to help. Note that I'm only talking about national radio, and only for listeners in the UK.
Read more and comment at BBC Radio Labs blog.
James Cridland is Head of Future Media & Technology for Audio & Music Interactive