As a facial plastic surgeon in Maryland, a good number of my patients are equestrians. The first and best answer, of course, is to check with your surgeon. Only he or she really knows what was done to your nose, and what it might be able to withstand.
Personally, I typically instruct my rhinoplasty patients to wait at least 1 month before trotting or galloping on horseback. Walking is fine at 3 weeks. The reason is that there are significant jarring-type forces that are conveyed to the rider while riding, and your new nose can suffer substantially. I'd worry less about a nose-altering injury if you've only had a tip rhinoplasty (cartilage modifications only), however, you're still vibrating the nasal tip and the fine sutures we use to secure our modifications can become dislodged or even break. Therefore, I think it best to allow your nose more time to heal. Starting at 3 weeks, I'll allow my patients to walk on their horses only. Then, at 1 month you can start trotting and galloping, providing there is no pain. A good rule of thumb is if something hurts, avoid it for an additional week. And definitely don't ride a horse that's known to be "spunky" or "spooks easily" for at least 3 months for fear of being thrown or bucked. Good luck... And see you at the Preakness!