As I understand it, (Meaning I could be mistaken) broadly it was twelve hours each, but they had some liberty to work things out between the two of them for things like meals or actually getting some sleep. Since it was only the two of them and they were expected to keep operating 24/7, this could still mean putting in some long hours. Especially if they had to make repairs to their rig or catch up in a backlog in traffic.
Since I'm in no sense an expert on these men or the way Marconi required them to do business, I'll defer to those who know the ground better then I do.
I think that the times were for Jack Phillips between 8 a.m. and 2 p.m., and 8 p.m. and 2 a.m., while Harold Bride's were between 2 a.m. and 8 a.m., and 2 p.m. and 8 p.m.. But of course these times were flexible, depending on how tired they were, what was going on around them, how much traffic there was etc.
I remember reading about watch hours in Harold Bride's testimony from one of the Inquiries. I'll have to check whether it was the British or the American one. In any case, there are full transcripts of both available on the Internet.
I recall that Bride said he relieved Phillips from 7 to 7:30 PM (I think) on April 14th so Phillips could eat dinner and was planning to relieve him again around midnight, two hours before he was "supposed" to. It seems like the shifts were determined by the operators themselves rather than the Captain or the Marconi Company.