How deep can discounts on Amazon get? I'm looking at the Hogwart Train set on Amazon right now and they are 20% off the MSRP. I recently bought the 60198 set at a 5% discount, now it is 16% off. 

I wanted to know typically how deep the discounts can go and how far into a release do they start discounting and how quickly they sell out. 

Can people who have a lot of buying experience on Amazon comment please?
As a general practise discount can be quite big (I've seen -50% off) for a small set and tend to be much lower (around -25% off) for a large set.

Discounts tend to vary even during the same day. So it is worth keeping an eye. Some sites, like Brickset tend to announce LEGO sales too. They even have a tracker for Amazon and other services. I suggest to look here to have a sense of understanding how much discounted the set can get.