Picture the scene: You’re in the middle of running a hectic game of Dungeons & Dragons, and the party is getting along well with Bunky, your ad-hoc monster NPC. Suddenly, one of the players springs an idea: “I wait until we rest, then kill Bunky.”
Suddenly, you need stats for your off-the-cuff creature. You have three minutes.
Rule 1: Generate only as much as you need.
Don’t bother calculating skill points, ability scores or anything you don’t think the monster will use. Some creatures only need a hit point total, attack bonus and damage, and perhaps Fort/Ref/Will.
Rule 2: Estimate!
Third edition has detailed and specific rules for how to calculate hit dice and ability score modifiers. You have three minutes. Borrow the hit points from a similar monster or guesstimate the stats; say, +10 to attack, 1d8+4 bite damage and 6×9 = 54 HP.
Rule 3: Stick to the rules
You have three minutes, not three seconds. Pay at least some attention to the way the game builds monsters. The third edition Monster Manual and fourth edition Dungeon Master’s Guide can give you some insights on this.