I have a client on a modified ab rehab program. She began with a large separation of 13 fingerwidths. In 10 weeks she had lost several inches and strengthened her connective tissue significantly, having lost 2 fingerwidths total, down to 11.
For 2 weeks she had no closure. Staying motivated despite this, she worked hard for another 2 weeks and lost 3 full fingerwidths! Now down to only 8 she is well on her way to a 2 finger "across the board" measurement, which gives her the green light to do (supervised & splinted) planks, pushups and all the other fun upper body/core stuff.
Stay motivated folks and take the small bits of progress as they come. Diligence and consistency is the key to closing your diastasis for good!