Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_203_875 ( id number, "#" number, "name" text, "birth and death" text, "office started" text, "office ended" text )

### Question ###
for how many years did hallgrimur sveinsson serve as the bishop of iceland ?

### Accurate SQL ###

SELECT "office ended" - "office started" FROM table_203_875 WHERE "name" = 'hallgrimur sveinsson'