I've never coached college football and can't imagine how complex things are during a game, but I've coached large school high school football and can tell you that while adjustments get made constantly, the vast majority are done right away. Most things (maybe 80%) you already know based on film. About 10% you figure out based on the first two drives...this is things like how a defense will line up against certain formations like trips, how their OL will pass block when you show certain looks on D, whether they're manning up or spying a top O player, etc. Based on that, you make a few adjustments and decide how you want to attack moving forward. The other team will undoubtedly make some adjustments as well, so you continue fine-tuning throughout the game.
In college they've got an infinitely larger pool of schemes to dip into depending on situation, so I don't doubt that the adjustments continue deeper into the game, but it's not like halftime is for miracle working. You might draw something up on the board that's somewhat "new" to guys (or that they didn't practice that week), but if it's anything like high school it's simply a time to talk with position groups individually to emphasize how the other team is attacking them and what they should expect in the second half.