The words "at all" could be removed from the sentence without any real change in meaning:
The biggest risk you will ever take is not taking one.
which would mean the same thing as:
The biggest risk you will ever take is not taking any risk.
In other words, if you are too cautious all the time, that becomes a risk in and of itself.
The line you ask about reminds me of another similar quote:
The only thing we have to fear is fear itself. (Franklin D. Roosevelt)
As Janus said in a comment, the meaning of at all is idiomatic, but you need to know to look for the phrase as a whole.
at all
• (used with a negative or in a question) in any way whatsoever or to any extent or degree ⇒ "I didn't know that at all"
• even so; anyway ⇒ "I'm surprised you came at all"
http://www.collinsdictionary.com/dictionary/english/at-all
The phrase at all usually emphasizes the text that precedes it.
Firstly: "o'clock" should not be used except after a number of hours, alone. "12 o'clock", yes; "12:01 o'clock", no; "noon o'clock", no. If you need to be clear that (for example) "ten fourteen" is a time, and not some other number, you could use "a.m."/"p.m." or say "fourteen [minutes] past ten".
Secondly: "on the dot", "sharp", and others are used to emphasise that a time is precise when people might think it's not. If I say "12 o'clock", you might well assume that I'm rounding to the nearest hour (or half-hour, or 15 minutes, or whatever). I could say "12 o'clock sharp" to be clear that I mean exactly 12 o'clock.
If I say "10:23", however, the fact that I bothered to specify a number of minutes, and not a nice round multiple of typical values, would suggest that I mean exactly 10:23. Saying "sharp"/"on the dot" would be redundant in this case, but not wrong.
That said, there are situations when even an atypical number of minutes might be taken as vague, and so "sharp"/"on the dot" would not be redundant:
When accuracy to within seconds is meant, and you want to be clear that you mean "10:23 and zero seconds". (Example: A school has morning break start at 10:23. The students want to be let out 30 seconds or a minute early, but the teacher is adamant that they will be let out at "10:23 on the dot".)
When atypical numbers of minutes are still only estimates. (Example: Bus timetables will give times like "10:23", but it's expected that it will likely be later. You might then tell a friend that the bus arrived at "10:23 on the dot", because you want to emphasise how unusual it was that it was there right on time!)
Lastly: "high" is (in my experience) only used with "noon"; not just the time "noon", but with the exact word "noon". The title of the movie you linked to, "Twelve O'Clock High", is not about time, it's about direction. In a military (or colloquial) context, directions relative to you can be specified by points on the clock: "12 o'clock" is straight ahead, "6 o'clock" is behind you, "5 o'clock" is behind you but a little bit to the right. And then aviators, having to deal with three dimensions, can also specify "high" or "low" (or, if I remember rightly, "level"). So the title "Twelve O'Clock High" is pilot-speak for "straight ahead and at a higher altitude".
Best Answer
What you are looking for is some hyperbole (say hi per bo lee), a type of figurative speech in which you exaggerate for a dramatic effect. No child is literally playing all month.
So if this is hyperbole, why not say "all day" or "every day". These are the usual expressions, unless there is some reason to specify that "months" are significant to the meaning. If you know that "all day" exists is there any reason to use "all month"
An alternative is to use
Which preserves the sense of "for many months"