When do you know if minoxidil is working?

If patients are not seeing results with topical >6 months into treatment, do you think a switch is advisable then?

If you are taking the oral, then you should see some benefit in 6-12 months or less. That may not be true for the topical because better than half of the men using topical get no response at all.  This is because for minoxidil to work, it needs a special enzyme , see here: https://baldingblog.com/sulfotransferases-and-minoxidil/