I've read three months to three years. Nerve sparing (or not) during surgery, rehabilitation programs afterwards, along with daily use of Viagra-like meds can make a difference. I've found these two sites to be helpful:

My RP was in March 2022 when I was 68. My surgeon suggested 20mg of viagra daily for a year once the catheter was removed. Stimulation as often as possible was also recommended. I started to have encouraging results in June 2022 but only when standing and only when taking a maximum dose (100mg) of viagra. In Sept 2022, I was able to have an erection suitable for intercourse, again only when upright and only with 100mg of viagra. My progress was interrupted when I began hormone therapy (ADT) for a biochemical recurrence in October 2022. I was still able to achieve an erection with viagra during ADT but desire was lacking and an orgasm was not possible. Now, more than seven months since the start of my six-month regimen of ADT and four months post radiation, I recently had an orgasm again and have had intercourse in a variety of positions. Still waiting for my libido to fully return, however. I was helped tremendously during the past 15+ months by my girlfriend who was and still is a very active partner. Best wishes for your speedy recovery.