First off, there isn't such a thing as a "magnetic pulse". A changing magnetic field is necessarily accompanied by a changing electric field, so you can have an electro-magnetic pulse. So your question doesn't make much sense.

>>How far would the to poles need to be from one another so that a single short pulse would require time to travel from one pole to another? <<

Any variation in a field would require time to travel from one point (pole) to another, no matter what the separation.

But to dig a little further in to what I'd guess you're thinking about, electromagnetic energy doesn't carry a lot of momentum (for any sticklers out there, I am speaking in simplified relative terms, of course). It seems extremely unlikely that an engine based purely on the notion of EM pulses would be viable. Gathering enough energy from sun light to power propulsion is also problematic, I think. You might want to read about solar sails and their limitations.