The optimal timing will depend on your target cell of interest within the PBMCs (T cells, B cells, NK cells, monocytes, dendritic cells), and may require some trouble-shooting. Your CpG stimulation will stimulate TLR9 on B cells and plasmacytoid dendritic cells, whereas your LPS stim should activate almost all of those cell types via TLR4.

This is a very nice methods paper (based on DCs) that goes through various stimulation times, when BFA is added (3 hours before the stimulation ends, although I've seen 2 hours before as well).  They looked at both CpG and LPS stim.

http://www.sciencedirect.com.ezproxyhost.library.tmc.edu/science/article/pii/S002217590500428X

In our antigen-specific stimulations, BFA is present during the whole stimulation time