Andy writes, "My parents arrived on visitor visas last month and within a few weeks I filed for adjustment of status for them since I am a US citizen and would love to have my parents here. Since I wanted their green cards to arrive before the expiration of their authorized stay on tourist visa, I rushed to do the paperwork, only later on realizing that once the application has been filed and an acknowledgement received, they will be in lawful status till the case is decided. While researching the same I also came across discussion that I should have waited for at least 90 days to petition them. What's next? Will their applications be rejected? Please help."
The rush with which you filed these immigration petitions makes it appear as if your parents never really intended to visit America as tourists (which, as you very well might know, is the intent of the tourist visa and whether the visitor is asked this question specifically or not either at the time of interview or at the port of entry, that is the implied intent). Because of the informally used 30/60 day rule, USCIS adjudicating officer will assess that intent of an alien was not leisure travel but to become a permanent resident. A tough decision maker at USCIS, who follows the rules strictly, will conclude that in order to submit the application, you probably started working on it as soon as they landed because it takes some time to organize all the paperwork and your parents probably brought along with them documents like birth certificates that a typical visitor would not -- most normal visitors do not just decide to settle in American within days of their arrival; it is a complex decision to abandon your country and make USA your new home (the rule of thumb, as you found out, is to wait at least 90 days after arrival). So, while a lenient officer might simply ignore all this and give a stamp of approval (knowing that they are most likely elderly and eligible), another agent may deny the application and ask them to seek consular processing after cancelling their visitor visa. It would be based on the simple logic that when they boarded a flight to America, they intended to stay there permanently (by the way, this is loophole that is so often exploited by immigrants that USCIS agents are fully aware of it). Immigrants would rather adjust the status right here in the USA than to wait overseas and deal with the nightmare of showing up at US embassies and consulates, which often require long travel and endless waiting in lines.
In addition, if this was their very first trip to the United States of America after receiving their B1/B2 visas, it might be concluded that they engaged in fraudulent misrepresentation to get those visas because they really did not intend to check out the Grand Canyon or Times Square; they basically wanted to take a shortcut to their lawful permanent resident status. In other words, they lied about their intentions to a Consular officer. Be prepared that this could result in revocation of their visitor visas and denial of adjustment of status. You could still appeal the decision and hope that an excellent immigration attorney (basically, you are paying the price of not consulting with a lawyer in the first place -- there is a reason why these folks study law) can convince a judge to overturn the rejection. Alternatively, you can do the paperwork again for them to go through consular processing.