I have kept my THP1 cells for weeks after differentiating with PMA. I did a study that required the cells to incubate for 14 days. I had to change the culture medium about every three days but I added the same concentration of PMA that I used to differentiate the cells. The only question that will remain is whether the gene expression will be the same after a prolonged incubation after differentiation.
Generally speaking, always differentiate the cells fresh for each experiment to take out the probability of differentiation error.