If you have recently applied or will be applying for a work permit (also called an employment authorization document or EAD) based on an application for asylum, and U.S. Citizenship and Immigration Services (USCIS) approves this application, your work permit will be good for two years. (See USCIS's announcement of October 5, 2016.) The previous validity period was one year.
This is no doubt in response to the backlog at USCIS and the difficulty it is having in scheduling people for asylum interviews and making decisions on their I-589 applications for asylum. (For more about this backlog, see Asylum Interview Delays: Beware the Dreaded Asterisk.)
How do you know whether your work permit is asylum-based? Look at the code you (or your attorney) chose to enter in Question 16 of Form I-765. If it says "(c)(8)," that means your eligibility was based on your pending or approved asylum application.
And if you're in the process of applying for asylum but have never applied for an EAD, be sure to read When Can Asylum Applicants Get a Work Permit?