SELECT (SELECT JULIANDAY("date of appointment") FROM table_203_156 WHERE "team" = 'crystal palace') - (SELECT JULIANDAY("date of vacancy") FROM table_203_156 WHERE "team" = 'crystal palace')
how long was there a vacancy at crystal palace ?
CREATE TABLE table_203_156 (
    id number,
    "team" text,
    "outgoing manager" text,
    "manner of departure" text,
    "date of vacancy" text,
    "replaced by" text,
    "date of appointment" text,
    "position in table" text
)