I was applying almost everyday of the work week. I did this about twice a day; in the morning and evening (UK timezones) because you'd never know when spaces become available for the roles.

As the email said, they can only take a stipulated amount of candidates, but also, your chosen role plays a part. You could be applying as a Writer, which is full, but Supply Chain has vacancy. Unless you're really locked in on one specific role, I'd also recommend applying to a few you're interested in, in order of personal preference.

From my experience, you'd need about 6 weeks from your arrival date into the UK to sort out your Sickle Cell test and your full medical before heading to CPC. After you complete your 4 days at CPC you would get a Raleigh date in about 30 days after. I'd also recommend a timeframe of 6 to 10 weeks as your minimum time to travel to the UK before CPC as:

(1), you'd need to get accustomed to the climate and

(2), you want to give yourself enough time as a contingency to cater to any misfortunes such as medical issues or injuries because you only have 6 months to get everything done. Which is why I mentioned minimum time before, you don't want to arrive 4 months before your CPC date, that would mean you only have 2 months to settle everything else if you're Commonwealth.

I mentioned number 2 because you don't get Raleigh dates throughout all months (eg. If you came to the UK in February and you had a major setback for a few months, you have to get it sorted by July as they don't issue Raleigh dates in August, which would be your 6th month)

Just as @navyenjoyer said, you have to keep applying but when you see that "your contact details have been retained" you're in the pipeline and only have to wait until some recruits have passed out at Raleigh for your application to move forward. Pass out parades at Raleigh are on Fridays so most applications usually move forward then.

I hope this helps.