Child support modification time limits

Forsyth county.
My children’s parent filed a motion to modify child support claiming 50/50 custody. Not true. I wonder if there are time limits that must pass before one can ask for a modification and how much time must pass under a new change in conditions. The original order went into effect around 5/2015. Mod request submitted 12/15. Thanks for any input.

There is no time limit on seeking a modification of child support, but the moving party must show a substantial change in circumstances. If she is claiming the change is 50/50 custody, and you contend that you have not been sharing custody 50/50 the you need to be prepared to put on evidence that you have not been sharing custody 50/50.