Slips, weaves, ducks, blocks, when do they come naturally?

[The BB of C]

They come naturally after training reflex, instinct, technique and learning to keep a clear mind and to "flow" with your opponent. If you train well and have good teachers and partners, it will come eventually. As for the question of 'when', everyone is different.

[NightOwl]

I guess the best thing is as simple as having someone try and punch you in a realistic manner (or trying to punch you period). You can learn the movement for a block or dodge all you want, however you need the real deal to teach you the timing and drill it into you until it becomes natural.