OPK - Help??

Does anyone know in terms of LH surge, does it stay constantly low and then just randomly one day spike? Or is it a slow climb that will start increasing each day? I'm on CD 8 or 9 and the lines on my opk's have been faint, as expected. I started testing with opk's early because I am just coming out of a phase of amenorrhea so I have no idea how long my cycles will be. Today I took one and it's getting darker. Appreciate any help!