the older he is when hes neutered the longer it takes for the hormones (and habits) to die back...a normal 6 month old neuter will uuslaly take about 2-4 weeks for the hormons to subside...
it also depends on the male, some are more stubborn and hold on as long as possible...
with a female in heat, so long as both testicles are removed cleanly, there usually ok around a female after about 2 weeks (full healing time) he may still try to mount and hump the female for few more weeks after that though.