Future Perfect Continuous Tense

What is Future Perfect Tense? How is it made?

The easiest way to understand Future perfect continuous tense, is to understand Future continuous tense.
There is just a little difference between them.
Future continuous tense is used to talk about an ongoing future action. But in future continuous tense, we don't mention or use time expressions or time duration. 
For example: 
I will be playing. (but since when?)
It will be raining. (but since when, and for how many hours?)
He will be working since morning. 

If you want to mention time expressions or time duration, then look at these examples:
* It will be raining. (no time period)
* It will have been raining since morning.
* By 2015, they will be living in Karachi.
* By 2015, they will have been living in Karachi for 5 years.
* He will be studying.
* He will have been studying for 2 hours.

When to use since and when to use for?

For is used with numbers and since without numbers.
* since morning * since evening * since 1990 
* since monday  * since July
* for 2 minutes/for 2 days/for 2 months
* for 2 years * for 2 hours

Sentence Structure of Future Perfect Continuous Tense:

Positive:
Subject + will have been + ing form of verb + since/for...?
It + will have been  + raining + since morning.
Negative:
Subject + will not have been + ing form of verb + since/for...?
He + will not have been + sleeping + since morning.
Interrogative:
Will + subject + have been + ing form of verb + since/for ... ?
Will + she + have been + working + since 2 days?