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.* 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.*1 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.*2
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.*2
Applicants can expect to remain on the U visa waitlist for three years or more until a visa is available.*3 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.*4 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.
* Information from ASISTA
*1 8 C.F.R. § 214.14(d)(1); see also USCIS website
*2 See 8 C.F.R. § 214.14(d)(2)
*3 Time approximation on waitlist is accurate as of 01/2018 as per ASISTA
*4 INA § 214(p)(3)(B)