Learn English – Heritage and Inheritance

They are mostly used in the same way. A slight difference is that there is a little more emphasis involved with 'that'. Then for most cases, "it" is used to address a single object,

We watched the stars at night. It was fun. (watching the stars at night was fun)

whereas, "that" can be used to address multiple things (not necessarily), or a group of things.

We went to the observatory and watched the stars. That was wonderful. (The trip to the observatory and watching the stars were both wonderful)

There are sentences where you cannot use "that", for example,

It was great running into you today.

Coming to your sentence,

• 'How long does it take?'
• 'How long does that take?'

You could ask someone :

'How long would it take for you to drive all the way here?'
or
'You'll have to drive all the way here. How long would that take?'.

Typically 'that' requires a point of reference. An event or an object, and that's mainly the reason why it is added to the second part of a sentence.

"Either" is a determiner, pronoun, adverb or conjunction. In the examples along with 'of' it is a pronoun occurring prior to the noun(pronoun!) it replaces and keeps both the options open but does not allow the liberty to choose both at a time.

In sharp contrast to this, EITHER is an alternative coordinating conjunction proceeding a word or statement followed by the disjunctive OR to emphasize the possibility of choice.

• Either come or write.

• You may choose either of them (the options).

No, in none of your examples you're not allowed the privilege of lighting them both at a time. You are given a choice.