Learn English – Is there difference between “by then” and “until then”

Fill means to add content to the container or gap until it is full.

In particular, "filling" tends to involve a physical action, such as filling a mug with water, or filling a form in with a pencil.

Please fill this jug with water

Please fill in this form.

I have filled in all of the empty days on the schedule with stuff to do.

We need to fill the hole in the road with new gravel

We need to fill the vacancy in sales before Tuesday.

Fulfil on the other hand means to complete an action or requirement. In particular, one tends to fulfil metaphorical or non-tangible things, such as requirements, objectives or promise.

You need to fulfil all of the requirements before the end of January.

Fulfilment of the key objectives is our primary goal.

I'll fulfil my promise to help you with your homework before it's due in, really I will!

There's nothing specifically wrong with any of those examples. There are three notable differences between using "until" vs. "how late":

1. Until is more formal and is soliciting a specific time, such as 3AM.
2. "How late" is less formal and an estimate is appropriate, such as 'sometime after 3'.
3. In these examples, "until" is not relative to anything. "How late" suggests someone was either working past their normal time or late at night.