Wah How to use "A While".

A while = A short time.
So if we say "I will be there in a while", it means "I will be there in a short time".

The opposite of a while would be a long time.

We can use a short while to emphasize the short period of time.

• It is impossible to master English in a while. (It is impossible to master English in a short time.)
• The system will go into operation in a short while. (The system will go into operation in a short time.)
• I'll wait here for a while. (I'll wait here for a short time.)
• Would you mind if I watch TV for a while?
• Let's sit here for a while and look at the view.
• Please stay out of my way for a short while.  
• This machine is out of order for a short while.
• Leave me alone for a short while. It's none of your business.

Wah How to use "Once, Twice, Three Times..."

If you want to say how often you do something, You can use...

    one time (once)
    two times (twice)
    three times
    four times
    five times, etc.

       Q: How many times a week do you go to the movies?
       A: I usually go to the movies twice a week. 

       Q: How often do you wash your hair?
       A: I wash my hair three times a week.

       Q: How often do you eat raw fish?
       A: I eat raw fish once a week.

You might hear the word once upon a time.
    According to OED, once upon a time means at some time in the past (used as a conventional opening of a story). 

What about a long time ago?
    It means a long time ago in the past. Now, they could be used interchangeably in some cases, but once upon time could refer to something that happened a few days or a month ago, which is not long time ago, while the other couldn't.