I dont see how you can possibly tell for certain that a person going into hospital now will be discharged on a given date, so I would award HB for the full 52 weeks from the outset.
Having said that, the issue of how long the person is likely to be absent (and continued entitlement) should be judged on a week by week basis (see CH/1237/2004)
The day of entering hospital and the day of discharge do not count towards the 52 week period, only complete days of absence (midnight to midnight) are relvant (R(S)1/66)
Exceptional circumstances for the purposes of awarding 52 weeks benefit where absence is likely to exceed but not substantialy exceed 52 weeks is not defined, but given that going into hospital for a year pretty exceptional of itself, it should not be too difficult to find circumstances to jutify paying the benefit.