SureFire recommends 60 lumens (depending on reflector size and beam characteristics) as the minimum output level to cause momentary blindness in a subject whose eyes are dark-adapted and who is no more than ten yards distant.
You might be tempted to choose a very high output WeaponLight for all purposes, but this could be the wrong decision. Higher light output WeaponLights use more batteries and are therefore larger and heavier. This can be significant when the firearm is used in tight quarters, or carried over long distances, or held in a firing position for extended periods of time, or must be manipulated with maximum rapidity.
Furthermore, in close range engagements, such as inside buildings or caves, too much light output can result in excessive bounce-back light that could degrade your own night vision. For close to medium-range operations, such as structure clearing, or where targets may be one or more blocks away or in upper-floor windows, a two- to three-battery (6V-9V) light is probably best.