New trades are added to your 30 day volume immediately, but trades older than 30 days are only removed every few days. This means that sometimes you might temporarily get a lower fee than you should, but you won't get a higher fee than you should.
Important: USDT (Tether) trades do not count towards your 30 day volume.
You can keep track of your current fees and progress towards lower fees by looking at the upper right corner of the "Trade" tab on your account screen.
"Current Fee" shows the fees you are currently being charged.
"Next Fee" shows the level your trading fees will drop to if you reach the next trading volume tier.
The progress bar in between these two fees shows how much more volume is needed to drop to the next fee level.
In the example above, you are currently paying 0.16% for maker orders and 0.26% for taker orders. If you trade a total of 50,000 USD worth within the 30 day rolling window, the fees would lower to 0.14% maker and 0.24% taker.
Remember, trading volume discounts are not permanent. If your trading activity drops, so do your discounts.