When you sign that contract you are signing a contract with the government. The alien cannot waive the obligations under that contract because they are only a third party beneficiary. Even if the couple ends up getting divorced.
Technically if you sign on as a sponsor you are saying that if the alien applies for and receives aid from the government in the form of welfare, food stamps, etc. then you will be held liable to the government for that amount.
The form you are signing should tell you how long you are held liable, which is usually until one of the following happens:
1) They become an American citizen (which through the marriage visa process could take as little as 2 years and 9 months).
2) Ten years pass.
3) They move out of the country.
I say technically, because in King Law Office’s almost three decades of existence we have never seen the government go after anyone for this liability, nor had any client of any fellow immigration attorney gotten in trouble with the government. If you google this topic however, you can see that the government has gone after some sponsors. However, this is usually the result of a lie told by the applicants for the marriage visa during the application process or a lie told by the immigrant to the government agency issuing the aid.