I was told 500 miles by a Ford Tech after an installation of gears and t-lok. That leaves plenty of time to heat cycle the metal and allow the friction modifier to wear the clutches evenly so nothing sticks.
Of course I didn't take this to heart and as soon as I got my car back I got on it from a stop on a country road and found me and my friend in a ditch with my car. Good thing it only cracked a foglite and cracked the bumper a bit. Gave an excuse for cobra upgrade
I've never had any problems though with no break in with over 100 1/4 mile runs, tons of street races and 1.8 60' times.
Its hard to keep your foot out when you know you just got something cool installed.
When the motor goes in I'm putting block of wood under the gas