You will need to have some sort of support for her in this prenuptial and the affidavit of support is not intended to last forever and if your soon to be wife later becomes a citizen then she would be able to support herself and your obligation ended.
The Form I-864 Affidavit of Support is a legally enforceable contract, meaning that either the government or the sponsored immigrant can take the sponsor to court if the sponsor fails to provide adequate support to the immigrant. In fact, the law places more obligations on the sponsor than on the immigrant -- the immigrant could decide to quit a job and sue the sponsor for support.
When the government sues the sponsor, it can collect enough money to reimburse any public agencies that have given public benefits to the immigrant. When the immigrant sues, he or she can collect enough money to bring his or her income up to 125% of the amount listed in the U.S. government’s Poverty Guidelines (as shown in the chart in Form I-864P).
The sponsor’s responsibility lasts until the immigrant becomes a U.S. citizen, has earned 40 work quarters credited toward Social Security (a work quarter is about three months, so this means about ten years of work), dies, or permanently leaves the United States. If the immigrant has already been living in the U.S. and earned work credits before applying for the green card, those count toward the 40. In fact, in marriage-based cases, work done by the U.S. petitioning spouse during the marriage can be counted toward these 40 quarters. A divorced immigrant spouse could decide to sit on a couch all day and sue the former spouse for support. The sponsor may wish to have the immigrant sign a separate contract agreeing not to do this, but it is not clear whether courts would enforce such a contract. Basically if she went on subsidy you may be liable to pay the government back, so you want be able to provide that she not go on government assistance if she has not met the criteria to release you and move back to her country. She should probably have an attorney present at the signing of the prenuptial agreement so that would rule out an attorney in her country.