I've put probes on escape trajectories from the sun several times in the past. Just dump in a lot of fuel and burn! (Mostly I do this in sandbox mode). In any case, I have warped at max speed for long periods of time and not found an edge to the sun's SOI. So it appears that the solar SOI is infinite. Also: SOI tends to be at the distance where the theoretical gravitational of a body looses out to another. Ie: if you use newtonian gravitation to calculate the gravity force to all the bodies around you, then the one with the largest force will the the one whose SOI the game puts you in. or: SOI in KSP = strongest real gravitational force. Given that the game doesn't model any other stars, it makes sense that, once you go far away from everything, the largest gravitational force is always just the biggest object.