Agree with mdh2020 that being bright alone will likely not be enough for many of the very-competitive schools. Most DC who are not in a prep school will need extensive preparation.
We went through this last year (North West London/ fairly competitive entries). DD was (still is) in state primary that sets no homework. She spent about an hour/hour and half every day, sometime a bit less, for prepearation (no professional tutor - we were teaching). Towards the later half of the year, in weekends, she spent a bit more time, about two and half hours.
We started from year five, which I now think is late - ideally we should have started end of year 4 and could have taken it slowly. She prepared at home with us, and started mock tests using Atom and few other online providers in the last two and half months (again, later than ideal)
Then there will be interview preparation too which I think is best done slowly over time. DD attended a number of interviews and underperformed in the first few due to nerves before gaining confidence. (the prep schools will do several interview practice sessions over months).
If we were doing it again, I wouldn't go with the DIY approach. It worked out okay in the end, but was lot harder and haphazard than it needed to be (due to our inexperience with the 11plus process as well as no prior teaching experience)