Some say only measure once every three months, as for official measuring.
I think as long as you're not obsessed with measuring, all is well.
That being said, you can measure after intense sessions for temporary gains, to see if you're going the right way.
Once a month is fine, for "semi-official" measuring. For best and most true measuring it's best to have a 48 hour timewindow without any PE to be sure of real gains. Some might say three days without exercises but damn it, PE is too fun to be put on halt for that long! LOL