You will definitely know when it's time. You will have your mind completely wrapped around it and you will do it!
It has been 2.5 years that I have been transitioning and in the past 3 weeks, I have found myself snipping relaxed ends here and there. (I've been wearing a wash n' go ponytail and haven't straightened in about a month so I haven't felt the shock of the shorter hair). I still have quite a few relaxed ends, but the fact that I have started doing it myself just tells me that I am nearing that time. (or either I am trying to lessen the blow for when I go to the salon so I won't have a heart attack at seeing a pile of hair on the floor
)
Fortunately for me, the two textures don't pose a problem with combing, but they DO pose a problem when styling. My relaxed ends ruin whatever wash n' go I think I may have (with all my hair down).
So I say all that to say that when you are ready, you won't have to ask anyone; you will find yourself either chopping it off yourself or either getting yourself to the nearest salon as fast as you can.
Good luck in whatever you decide (and make sure to post pics!!)
BB