Well this is something that will only really come with practice, specific punches are blocked in specific ways.
When fighters are taught to block and parry in our gym fighters take turns in either throwing or defending against punches we first go through the motions slowly so that the newer fighter gets a feel for the blocks and also the other fighter gets to focus on correct technique when throwing the shots, as time goes by you start to see these punches coming and start blocking them without even knowing you are, this is when your defensive techniques start to become more like second nature to you.
How long it takes depends on how long you drill for. Mike tyson has amazing reflexes and many try to imitate his slipping, bobbing and weaving style with little success, it is said that Cus Damato (Tysons trainer) made Mike slip, bob and weave against his sparring partners and not throw a single punch for 2 years, also if any of you have seen mike on his slip bag (small tear shaped bag full of rags or sand about the size of a hand that swings rapidly) you would know why his defensive skills were so good.
Another method other than blocking and parrying are corner drills, one person stands in a corner whilst another throws shots at them, this will help develop head movement and defense in general and will help kill the habit of flinching when being punched and also getting used to being pressurised.
Range is another important thing, you should know what punches to deal with in what range you are in (close = hooks, uppercuts. medium = crosses, jabs, Far = jabs ect)
Finally telegraphing, if you opponent telegraphs you will be able to see things coming, an old saying in boxing is to read the hips, if you do you will know that a punch will be coming from the side thats moving.
Hope it helps,
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