Single word for the "estimated time of completion"

You might consider the word duration.

As Dan mentioned in comments though, it's quite acceptable to use ETA for just about any "estimated times".

How about time frame?

OD:

time frame: A specified period of time in which something occurs or is planned to take place.

Completion Date (/time)

Often, in projects ‘completion date’ is used to denote the estimated (or actual, once complete) moment for the completion of a task.

Example: When is the completion date for room 3 curtains and trimmings? - Next Wednesday around 4pm.

You can abbreviate it to ‘complete date’ if you like.

Other words:

Start date, end date. Start time, end time. Target date, actual complete date, etc. Usually what each of these specifically mean is defined at the start of a project, so that everyone uses consistent language, to avoid confusion.

Note: quite a few project websites use ‘finish date’. I much prefer ‘complete date’. Because then you avoid discussions about ‘well, how finished is ‘finished’? Whereas complete alludes to the goal ‘the item is complete’ rather than the process ‘we need to finish it!’ And in order to succeed, one should be aiming for ‘completed goal’ not ‘a story about the process’.

https://www.lawinsider.com/dictionary/project-completion-date

"Drop" is used in some industries to denote when something is complete, such as in software development. You can say of a software project that "the next version drops in two weeks" to mean that it is expected to be released and therefore "complete" (at least to some definition of "complete" for that version) in that duration. People sometimes ask "how long until version X drops?" with similar meaning. From that you could use "drop date" perhaps to refer to a specific completion date, if two words are acceptable.

In military terms, ETC means estimated time of completion. Don't use ETA if you are already there.