Republican presidential nominee Donald Trump has said a lot about immigration. During a Republican debate last year, he said: "We have a country of borders...We have a country of laws. We have to obey the laws. It’s fine if they come in, but they have to come in legally.”

People, including Trump, think it's easier to immigrate to the U.S. legally than it really is, but it actually takes a lot longer than most people think.

As law professor Neils W. Frenzen, director of University of Southern California's immigration clinic, told ATTN:, "This is a largely held belief, but it's based upon a complete lack of understanding. It assumes there is a path for everyone, and there is not."

In reality, the length of time the immigration application process depends under which category you fall. An application can take anywhere from a few months to several decades to be approved.

Here are the categories of people who may legally immigrate to the U.S.:

• Immediate relative and family sponsored, which includes subcategories for the spouse of a U.S. citizen, the fiancé to marry a U.S. citizen and live in the U.S., inter-country adoption of orphan children by U.S. citizens, certain family members of U.S. citizens, and certain family members of lawful permanent residents.
• Employer sponsored, including employment-based immigrants (such as professionals holding advanced degrees and persons of exceptional ability, professionals and other workers, employment creation and investors), religious workers, Iraqi and Afghan translators/interpreters, Iraqis who worked for/on behalf of the U.S. government, Afghans who worked for/on behalf of the U.S. government.
• Diversity immigrant visa

• Returning resident

The fastest route is for people with immediate family who are U.S. citizens. Frenzen says this is the one category of family sponsored visas not subjected to quotas and for which there is no waiting list.

"In the most common situation [of legal immigration], with the intending immigrant outside of the U.S., bureaucratic delays for processing at different U.S. consulates in different countries is going to vary a little bit," Frenzen said. "This would likely be within a year, assuming there are no problems that arise. But that's always a possibility, because the process can be complicated."

According to Frenzen, common obstacles people face include having a criminal history or problems with identification and "the public charge ground of inadmissability." As U.S. Citizen and Immigration Services (USCIS) defines it, a public charge means "an individual who is likely to become primarily dependent on the government for subsistence, as demonstrated by either the receipt of public cash assistance for income maintenance or institutionalization for long-term care at government expense."

So how does someone avoid landing in this category? The person's sponsor must prove that "he or she has an annual income of not less than 125 percent of the federal poverty level," according to USCIS. Frenzen points out that the family size of the sponsor is factored in as well. "[The sponsor] has to have adequate income to show they could support current family and those coming to U.S.," said Frenzen. "This is not a problem for a rich or middle class person, but for the working class or those with a lower income, [immigration] may be impossible as a result."

Another quick route to an immigration is the employer-sponsored category. According to the legal site NOLO:

"The cutoff dates and wait times for employment-based immigrant visas tend to be more favorable, but the tradeoff is that the underlying employment-based petitions require a high level of work and documentation from both you and a sponsoring employer. The employment first preference category, reserved for “priority workers,” is available only to workers who are considered outstanding in their field, such as internationally recognized artists, award-winning scientists or the like, or executives or CEOs of multinational companies. Historically, there has been no wait to get a visa in the first preference category."

The wait for an immigration visa can be the longest for relatives who don't fall into the immediate family category.

"The longest quota backlog is for brothers and sisters of U.S. citizens seeking to immigrate from the Philippines – a 23-year backlog," said Frenzen.

According to Frenzen, the people on that backlog were essentially put on a waiting list after the Philippines met its quota each year over the past 23 years. Why?

"There are not different quotas for different countries," said Frenzen. "The reason there's a greater backlog among Filipinos relative to Canadians [for instance] is simply, because there are more Filipinos with siblings who want to immigrate U.S."

And those Filipino siblings are not alone. The "line" for legal immigration is 4 million people, reports NPR.

But at least they have a path to immigration. According to Frenzen, a large number of the 11 million undocumented immigrants in the U.S. are low-skilled workers who do not have a high level of skills, graduate degrees, or a willing employer — things that may be necessary to begin the process of obtaining a legal immigrant visa, Frenzen said.

Here's why immigrants typically come to the U.S. illegally.

"Common reasons are family relationships and employment opportunities. There's also the concept of forced immigrants," said Frenzen. "Why are Syrians immigrating to Europe? Why are kids leaving Honduras and El Salvador? They're facing dangerous, life-threatening situations, fleeing poverty and starvation. So it's about survival – it's the only way they can continue to live."