In Star Trek, especially during the Next Generation era (TNG, DSN and VOY), we frequently see people having real time, lag free video communications with people in other solar systems (for example, from Deep Space Nine in the Bajoran system to Cardassia Prime in the Cardassian system).
My question then is, assuming no outside interference and zero signal degradation is there any lag factor involved with subspace communications? If so, how far apart would the end points need to be for this to become an issue in practice (where it would be easier to send a written message rather than deal with the lag in a video conversation)?