Writers of Star Trek often make the distance intentionally ambiguous to prevent know-it-all fans from screaming canon-violations. But we can presume its not all that far because... 1. We know Vulcan is only 16 lightyears from Earth. One would think the ancient Romulans leaving Vulcan could not have gone too far (thought this could be incorrect, especially if their migration was assisted by an alien species) 2. They fought a war with Earth in the late 2150s when Warp Drive was not that well developed.