Susan,

I am sorry for the pain that you are going through. This is a painful disease for everybody who is involved. You really can't tell how long a person has. My father in law suffered from Hotchinson lymphoma first for 5 years, and developed the lung cancer a year ago. He was a strong, brave man. He ended his treatments about 4 months ago because his body couldn't take anymore.
Ask your doctor what he thinks timewise. He will probably give you a range. We knew as of 6 months ago that he only had 3-6 months to live.
You should get his affairs in order now. Just in case he goes earlier than you expected. I know this is difficult to say, but you should also discuss with him about whether or not he wants to sign DNR (do not resesitate). It should be his choice while he is still in sound mind.
Also, you said that your mom will have a hard time working. Does she have any friends that own any businesses that may be able to offer her a job? Or possibly if there are any elderly people in your community looking for a companion. Many elderly people pay someone to stay with them through the day. This is a job that she would not have to be on her feet, and she would also be getting some companionship.
Best of luck to you, and hang in there. You need to be strong for yourself and your mother. Put your faith in God.
Keep in touch.
Kassy