Ophiopogon japonica - not true aquatic plant,it will die in couple weeks...
They are not being honest with you.Just as an FYI, I noticed O japonica being sold at petco as an aquatic plant, so I contacted the company to complain about that practice and they replied that their O japonica is grown completely under water from the start so that it adapts to the aquatic conditions.
If that is correct, it may be able to live underwater for a very long time. Actually if their implication is correct, it may not die at all, despite it being a non-aquatic plant in its usual habitat.
I don't know if they are right, just passing it along as they gave it to me. Perhaps you can reply later in this thread and update us with its status.