"Tomorrow and tomorrow and tomorrow" has eleven syllables. The effect of the extra syllable at the end of the iambic pentameter line may be to suggest that the "tomorrows" continue to be repeated in Macbeth's mind, as:
Tomorrow and tomorrow and tomorrow and tomorrow and tomorrow and tomorrow and tomorrow and tomorrow...
Note that Shakespeare plays on the extra syllable in the first line when he refers to the "last syllable" of recorded time. It is as if all the "tomorrows" are trying to catch up with that last syllable "ow" at the end of the first line.
