cua-prefix-override-inhibit-delay is a variable defined in `cua-base.el'.
Its value is

If non-nil, time in seconds to delay before overriding prefix key.
If there is additional input within this time, the prefix key is
used as a normal prefix key. So typing a key sequence quickly will
inhibit overriding the prefix key.
As a special case, if the prefix keys repeated within this time, the
first prefix key is discarded, so typing a prefix key twice in quick
succession will also inhibit overriding the prefix key.
If the value is nil, use a shifted prefix key to inhibit the override.

You can customize this variable.