Depends on the time of day and what mood the Spanish authorities are in.
Sometimes it can take 2 hours, sometimes you're straight through.
There's a side issue at the moment. The local Government employees in La Linea (the border town for those that don't know) haven't been paid since April (sounds familiar). The unions and protestors have staged demo's at the Border which closes it for a period of time.
There's an awful lot of sympathy for these poor guys and girls.
Causes all sorts of problems in Gib though. It doesn't take long for grid lock to snarl up the whole of Gib when the Border is closed.