Bosian theory has two waves, the retrospect and the aspect, the retrospect is a theoretical wave that would travel backwards through time from the point it was originated in, this theorum links time and space....
Particles that could be effected by this retrospect wave, would be able, in theory, to transverse through any point in the space time continumm (providing it's already happened).
According to current science the retrospect doesn't do much, however in theory it could be used to dictate the time at which something would 'arrive' after its 'left', the aspect is a more physical wave that would dictate where the things would arrive, however with current physics models it also dictates the time (because obviously you have to travel through space which takes time so the aspect will control how long it takes to arrive at the vectored exit point), normally the aspect 'controls' and 'covers' the retrospect because aspect waves are much more powerful...
I'll need to re-read some stuff before I can explain it anymore then that...
Sorry, it's been a while....lol
But yes, you're right, for plots sake at least subspace comms must be real time, the point is, if the subspace drives use bosian theory, they could arrive instantaneously, or in a hundred years, and the only difference to the people onboard the said driven ship would be what they noticed when they got out the other side...
....in theory though, it's instantaneous (intersystem) for the people who're in subspace, it's just not so instantaneous for everyone else.....