CREATE TABLE table_203_156 (id DECIMAL, "team" TEXT, "outgoing manager" TEXT, "manner of departure" TEXT, "date of vacancy" TEXT, "replaced by" TEXT, "date of appointment" TEXT, "position in table" TEXT)

how long was there a vacancy at crystal palace ?
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')
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')