15 min walk for each.
Because the city itself is rather small, pretty much everything is within walking distance. The existing maps don't really convey how close everything is. I can walk from my house, which is slap bang in the city centre (2 minute walk from the cathedral) to St Lukes in about 15 mins (a regular walk for me!).
Edit: I should also say that the walks themselves can be pleasant. The walk from St Lukes to the city centre (through Magdalen Street) is leafy and littered with quirky shops and cafes (it's probably the most expensive part of Exeter - St Leonards). The walk from Streatham to the city centre (past the clock tower) is also very attractive, and also full of little shops and pubs half way there. The main thing to worry about is the size of the main campus and where you will be living/studying in it!!! My GOD, if you are based on the far end of the campus you'll spend just as much time walking to your destination as you would getting to the campus itself!.