I would say as a rule of thumb that if you take 20 mins or less to do a Bmech assess then you haven't done it properly.
On the other hand I've had people come into my clinic, without an appointment, in a rush and a tizz because they have excruciating foot pain and they've gone out 10 minutes later pretty much pain free. E.G. they have PTTD and a max pronated foot, I nip out to the workshop, grab an old orthotic with a medial skive that's about the right size, try it out for comfort, maybe add a soft top cover to smooth out the fact that it wasn't made for their foot and Robert's your mother's brother.
So horses for courses