You can tell if someone is definitely not up to it by Christmas. No doubt about it, that is enough time if the manager is rubbish. Some weak managers can fool you a bit longer.
Better managers, you need to give them at least a season to know if they fit, and often after about 18 months you find they are flattering to deceive.
Usually, a really good manager is obvious within a fairly short time, certainly with a season.
I tend to decide on a manager quite quickly because I’ve seen many come and go now over the last 30 years. There are often obvious signs with weak managers and similarly with great managers, so i hope to see the latter from him in the first dozen games. But I’ll give him at least till Christmas.
The thing is that when Poch first came it wasn’t very good so realistically he gets one season at least