The show tends to have 2 or 3 "acts". Each act beings or ends with a dramatic or plot altering event.
As an example, season two had the nuke plot as Act 1 for 15 episodes and the conspiracy as Act 2 for the remaining.
Season 4 had three acts. Act 1 was the nuclear meltdown. Act 2 was the attack on Air Force One. Act 3 was the nuclear bomb/missile. Each "act" was roughly 8 episodes.
Season 5 has been similar. Act 1 was Jack coming back, main characters getting whacked, and the terrorists getting the Sentox. This was roughly seven episodes. Act 2 was the release of the Sentox at the mall and CTU, Tony joining Michelle in CTU heaven, and then the gas works. On Monday act three beings.
I remembered a while back on the thread both of ya'll were posting Chloe pics or pinging to them, I thought you might like these.