One way is to replace capital letters with space capital letters then make the first character uppercase:
var input = "itIsTimeToStopNow";
// add spaces, lower case and turn into a char array so we
// can manipulate individual characters
var spaced = Regex.Replace(input, @"[A-Z]", " $0").ToLower.ToCharArray();
// spaced = { 'i', 't', ' ', 'i', 's', ' ', ... }
// replace first character with its uppercase equivalent
spaced[0] = spaced[0].ToString().ToUpper()[0];
// spaced = { 'I', 't', ' ', 'i', 's', ' ', ... }
// combine the char[] back into a string
var result = String.Concat(spaced);
// result = "It is time to stop now"