See Table 1 in this article (http://www.thermofisher.com/content/dam/LifeTech/migration/en/filelibrary/support/bioprobes/bioprobes-65.par.61751.file.dat/bioprobes-65-cellevent.pdf) where a Jurkat model system was induced with 10 µM camptothecin for time periods of 0 to 4 hours. It is important to note that these results were studied using a single cell type and induction system; results may differ for other experimental systems. Increased membrane permeability in apoptotic cells can be discriminated from dead cells using YO-PRO-1 dye in combination with propidium iodide or the SYTOX dead cell indicators. Other mid-apoptotic events are increased ROS production (detected with CellROX reagents, H2DCFDA), changes in cellular pH (BCECF, SNARF-1) and calcium release (Fluo-4, Fura-2, Indo-1). No single parameter defines apoptosis under any condition, so it is best to employ a multi-parametric approach when studying apoptosis.
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