How long does it take to get the U visa? What legal status do I have while I am waiting for my U visa?
From the day that you apply for a U visa until you actually have the U visa in your hand, it could take up to 5 years or more. This long delay is for two reasons. First, there is a processing delay for U visas and so the US Citizenship and Immigration Service (USCIS) won't even look at your application for a few years. As of January 2018, USCIS is currently reviewing applications that were filed in August of 2014, which means that there is a nearly a 3½ year wait before USCIS is even looking at the applications that are filed.1
While you are waiting for your U visa application to process, you do not have legal status and could be subject to detention or even deportation. If you are in detention or in deportation (removal) proceedings while awaiting a U visa, Immigration and Customs Enforcement (ICE) agents and lawyers will look at the “totality of the circumstances” to decide whether a stay of removal or terminating the removal proceedings is appropriate.2 For more information, see Can I apply for U visa status if I am in removal proceedings (deportation)?
The second reason for the delay is that USCIS can only grant 10,000 U visas per year, which is commonly known as the "U visa cap." Once USCIS grants the 10,000 applications, they cannot issue any additional U visas for the rest of the calendar year.3 However, USCIS does continue to work on U visa applications that have been filed. If an applicant would be eligible to receive a U visa (but can't get one since the cap has been met), USCIS places that "approvable application" on a waitlist until it is his/her turn to be issued a U visa.4
While your approvable application is on the waitlist, USCIS puts you in "deferred action" status. Deferred action is not actually a legal status but it means that USCIS knows that you are in the country and you are eligible to apply for a work permit, which lasts for two years but can be renewed.3
Applicants can expect to remain on the U visa waitlist for three years or more until a visa is available.5 Once you get your U visa (if it is ultimately approved), you will get a four-year work permit since the U visa duration is a four year period.6 After you have had your U visa for three years, you can apply for lawful permanent residence (your "green card") if you meet certain requirements.
1 Information from ASISTA
2 See ICE Directive 11005.1, as explained on the Revision of Stay of Removal Request Reviews for U Visa Petitioners Fact Sheet
3 8 C.F.R. § 214.14(d)(1); see also USCIS website
4 See 8 C.F.R. § 214.14(d)(2)
5 Time approximation on waitlist is accurate as of 01/2018 as per ASISTA
6 INA § 214(p)(3)(B)