Journal #8216

Half-Life 1: November 19th, 1998 (2 years after Valve's founding in 1996)
Half-Life 2: November 14th, 2004 (5 days short of 6 years later)
HL2 Episode 1: June 1, 2006 (19 months later)
HL2 Episode 2: October 10, 2007 (16 months later)

On October 6th this year we will have passed the longest time between official Half-Life releases.

Now, some crazy speculative mathematics:

Half-Life was completed in nearly 2 years / 24 months. 24 divided by 4 equals 6. Half-Life 2 was developed in nearly 6 years / 72 months. 72 divided by 4 equals 18.
Half-Life 3 will be developed in 18 years (After 2004) so will be released in 2022.

Which means unless Valve does more episodes for HL3, we won't be seeing HL4 until 2076, and if the Fallout Series is to be believed the world will fall to nuclear devastation in 2077. I wonder how many mods will be released before the apocalypse...