CREATE TABLE table_204_625 (id DECIMAL, "model" TEXT, "availability" TEXT, "introduced" TEXT, "length of use" TEXT, "last of whiteness" TEXT)

crest 3d intensive professional effects and crest whitestrips 3d professional effects both have a lasting whiteness of how many months ?
SELECT "last of whiteness" FROM table_204_625 WHERE "model" = 'crest whitestrips 3d professional effects'
SELECT "last of whiteness" FROM "table_204_625" WHERE "model" = 'crest whitestrips 3d professional effects'