For April 18, 2015 test:
If you are form U.S :
You have until March 13, 2015 to register for the April test. March 20th if you are okay with paying the late fee.
You can register for it now if you like.
If you are outside of U.S ( including U.S territories )
you have until March 6th for regular deadline and 13th for the late.
Physics Subject GRE is offered only three times a year and the dates are given in the above link. Good luck