Visitors visa for parents of F1

Hello
I am a student on F1 since 2 years .My parents are applying for visitors visa. I read the documents needed . But there is one problem . I will not be able to give sponsorship letter since I am a student . I work on -campus part time and that salary is not enough to sponsor anyone . Finance is not a problem since my parents will be paying for their whole stay here . Can we tell the visa officer that they will pay for their stay or will he compulsorily ask for sponsorship letter? I have already sent an invitation letter to them .

Additional info : Both are working in India .Mother in govt service , father in private company. They have a house in their name .

My questions
1) What will happen if they do not have a sponsorship letter ?
2) Visitors visa is said to be 3 months to 10 years . For how much duration will they get it in this case ( since I am not on H1 ) ?
3) Since they are both working , they will come for around 3 weeks . Should they tell the visa officer that ? Will that affect the duration of their visa ?

Thanks