They vary, but a lot of the time they spawn at the top of the board. If you check the level pages, I'm sure you can find out where they start to spawn in. Interestingly, there are also 2 levels (129 and 192) that don't even spawn candies! Hope this helps.
Yes, i do know that they vary from level to level. How the new candies are spawn in each level is the question. Or should I say the order in which these new candies are spawn, is there an algorith for that? And thank you, i totally forgot about the level where candies dont spawn. :)