This reply certainly came a lot quicker than Prince William's birthday reply this year, which was a unheard of 4 months, which is the longest response time I've received. Clarence House had been backed up most of the summer, with the huge number of correspondences for Princess Charlotte's birth and then taking time off for summer holiday.
This reply was mailed on October 30th, 1.5 months after Prince Harry's birthday. Last year's reply is postmarked October 3rd, so quite a bit quicker, but the delay in their years response is understandable given Clarence House's busy summer and still falls into the normal response time.
The reply on the back of the card is your standard birthday message.
Prince Harry + Cambridges responses