How long does it take a hopper to deposit ONE item into a chest?

It will take 8 ticks to deposit one item. It will take a different time to Deposit into other kinds of chest though.

From the wiki:

Hoppers have a "transfer cooldown" time. After pulling and/or pushing items, a hopper waits 4 redstone ticks (0.4 seconds, barring lag) before pulling or pushing again (a transfer rate of 2.5 items per second, barring lag).
_{https://minecraft.gamepedia.com/Hopper#Redstone_component}

That means that the time to deposit one item is actually 0 seconds (it disappears pretty much instantly out of the hopper when you put it in manually), but depositing 2 items takes 0.4 seconds (or 8 game ticks), depositing 3 items takes 0.8 seconds (or 16 game ticks) and so on.

The wiki says 8 ticks, but I just checked it with the Carpet mod in 1.16.3, and for me it is 7 ticks.