Theory:

The terms FOR, SINCE, UNTIL and AGO are used to talk about time, but the way they are used is different.

EXAMPLE:
It is now 10.00
I started playing tennis at 8.00
I intend to finish at 11.00
So the amount of time I have spent playing tennis is 2 hours.

I can describe these 2 hours in the following ways.
I have been playing tennis FOR 2 HOURS. ( this form describes the amount of time spent.)
I have been playing tennis SINCE 8 O'CLOCK. ( this form tells you when I started.
you know the time now, so you can calculate how long I spent playing tennis.)
I started playing tennis 2 HOURS AGO. ( again, I define when I started, and let you calculate how long I spent playing tennis.)

I can also say I WILL BE PLAYING FOR 1 HOUR
or I WILL BE PLAYING UNTIL 11.00

FOR is used for a period of time like 2 hours, or 6 days.
EXAMPLE: I am going to study FOR 3 HOURS.

SINCE is used to identify a point in time.
This point usually indicates when the activity started.
EXAMPLE: I have been studying SINCE 7.30pm

AGO is also used to identify a point in time, by using the amount of time spent and going backwards.
EXAMPLE:
If I have been studying FOR 2 HOURS.
The amount of time I have spent is 2 hours,
So I started 2 HOURS AGO.
I use the period of time to identify the point in time when the activity began.

UNTIL is also used to identify a point in time, like SINCE, but SINCE is used for the past and UNTIL is used for the future.
EXAMPLE:
I will work UNTIL 6.00 ( This defines the moment in the future when I will stop working.)

FOR can be used with all tenses:
He sleeps FOR 6 hours.
He is working FOR 2 hours today.
He has been talking FOR a long time.
You slept FOR 6 hours.
Will you need the car FOR more than 2 hours tomorrow.

SINCE is normally only used for the PERFECT TENSES (have, and had)
EXAMPLE:
I have been talking SINCE 11.00
I had been busy SINCE 9.00 when I saw him.

AGO is normally used with the PAST SIMPLE.
It is also normally put at the end of the sentence. EXAMPLE:
I saw him 5 hours AGO.

Until is usually used for the FUTURE SIMPLE.
EXAMPLE:
I will work UNTIL 6.00

BY is used to define a specific moment in the future.
EXAMPLE:
I will be finished BY 6.00.

DURING needs to be used with care.
It can sometimes be used by Spanish speakers to mean FOR.
They are not the same.
It is wrong to say I HAVE BEEN SLEEPING DURING 3 HOURS.

DURING is used to identify an activity within another activity that has been defined.
EXAMPLE:
Last year I went to Paris FOR 3 months.
DURING this time I met Maria.
DURING places the activity of MEETING MARIA within the period FOR 3 MONTHS.

FOR is normally used with a question form of HOW LONG.
EXAMPLE:
HOW LONG has he been sleeping?
He has been sleeping FOR 3 hours.

SINCE is normally used with a question form of SINCE WHEN.
EXAMPLE:
SINCE WHEN has he been sleeping?
He has been sleeping SINCE 8 O'clock.