Success rates are often very individualized, so these numbers are difficult to provide.  After a patient comes in with his partner and has a full evaluation, including history and physical exam, a detailed expectation of success rates can be provided. Success rates can be defined as either sperm coming back to the ejaculate or pregnancy, and most success rates in the vas reversal literature refer to the sperm only. On average, at 5 years, around 95% of males will have successful return of sperm to the ejaculate. At 10 years, it’s around 80%. Even when the vasectomy interval is over 15 or 20 years, the success rates of return of sperm will be lower, but they remain around 50%.
Patients should really keep in mind that these percentages are not pregnancy rates. Pregnancy often occurs after a successful vas reversal, but only in the setting of a fertile female partner.  Good prognostic factors on the female side are prior pregnancies, normal menstrual cycle, age less than 40, and normal AMH levels indicating good ovarian reserve.
Pregnancy rates decline with longer vasectomy intervals for various reasons, including female factor infertility combined with lower sperm counts even in reversals that were successful.
For example, if a man has had a vasectomy for 15+ years and undergoes a reversal that is successful, the sperm count might come back low for a variety of different reasons (he is 15 years older, other medical issues have come up in the interval, or simply the testicles are not making as much sperm). If his female partner is close to 40 years of age or has other causes of infertility, it can still be difficult to conceive despite the successful reversal. However, if this same man’s female partner is 25 years old, it is quite possible he will have a successful pregnancy even with his low sperm count.