--Gabapentin has a half life of somewhere between 4 and 8 hours, with considerable individual variation, at least partly because unlike most drugs it is not appreciably metabolized by the liver but excreted almost whole through the kidneys--one reason one needs good kidney function to use this or related drugs (such as Pregabalin).

Now, there is a more time released version of gabapentin--Gralise--that is formulated to last somewhat longer; a lot of people have reported trouble getting insurance companies to spring for it, though.