Likely very similar to a lot of other passenger liners of the period. Wartime service as a troop transport or a hospital ship, regular runs in peacetime, and assuming the ship survived to World War Two, service again as a transport.
Kevin. I believe similarly to Michael. However, if you look at her sister's track record, I have a feeling that Titanic would have gone to the scrapyard with Olympic when Cunard and White Star merged, not making it to WW2.