I have had some horses gelded at varying ages.
Sundance, Eclipse, and Skipper all gelded at 11 mos and no studdish behavior had been established.
Jack, gelded at 6mos and a yearling now but he mounts and appears to be really breeding mares if he gets the chance, but other than that does not act studdish.
Derby and Lonesome, gelded as 5yo and 4yo stallions, it took about 4 months to see all their studdish behavior leave, but the majority of that was gone in about 2 mos. The vet told me to wait 1 month to put them in with mares, but I have yet to put them in with mares (could, but no reason to).
And, for the record, I have not ever regretted having a horse gelded. After Derby was gelded, he had some temporary complications (bleeding and had to be packed and on stall rest), but even then, I knew once that was behind "us", he'd be a happier and probably even healthier horse for the rest of his life.