The average decision time of math PhD application (Brandeis, Tennessee, York, SFU)
For most American grad schools, April 15 is a significant date for admittance offers. 

Here is the relevant link about the April 15 deadline. In essence, students who receive offers for grad school admittance are allowed to wait until April 15 to respond. What can happen is that students who receive offers sometimes wait until the last minute to decide, and thus the grad schools don't know if they have enough slots to offer someone further down their waitlist until around April 15.

Thus, I think the latest you might get an offer from a grad school is April 15, or perhaps a day or two later.