15 minutes is at the very limit of turn-up-and-go, and frequencies are often x20 or x30 on parts of LO, particularly in the evenings.Here's a supplemental question....
Bearing in mind that my local station is Wressle, with occasional bouts of Howden and Selby, it always seems odd to me that they bother with passenger timetables at all down in that London. When trains are going every ten to fifteen minutes, why not just turn up and hop on the next one (which is kind of how I feel the tube works, even though I assume there is some sort of operational timetable somewhere).