Do not let your cells rest in PBS for more than 20-25 mins.They will lose their adhesion molecules and half of them wont attach to plastic especially the MSCs.
usually cell lines are not kept in PBS for longer time not more than 5mins to maintain them in healthy state. If you are fixing the cells then it is ok to keep it for 20-25mins.
This depends on the purpose. I know few of my colleagues who collects the sorted cells in PBS at 4oC. I too stain the cells Hoechst in PBS without fixing for 15 minutes. Also for peptide uptake studies cells are incubated in PBS supplemented with 1% glucose for one hour or so.