The guidelines for allowable conditions for repair are as follows.
The "Driver's Primary Viewing Area (DPVA)" or "acute area" is that part of the windshield directly in front of the driver (which of course varies with the height of the driver so has to be taken as centred in front of him or her) 12" wide and at the top of the wiper sweep. For obvious reasons this area is crucial for the driver and must be kept as clear as possible. Therefore, repair is permissible if the damage can be completely covered by a 25 cent US coin, if the finished pit will be less than 3/16" across and if there will be minimal cosmetic blemishes remaining in the damaged area after the repair is completed. If the repair is within 4" of another repair, it should not be undertaken. Cracks running through the acute area should not be repaired; the windshield should be replaced.
If the acute area isn't involved, repair is not recommended under the following circumstances:
This doesn't mean to ignore the damage — it means "replace the windshield"!
These conditions may seem a little onerous. It must be remembered, though, that the windshield is under considerable stress to begin with, from the requirements of installation, from air pressure at speed, and from thermal expansion and contraction, and its integrity and strength can be lost very suddenly if damage is incurred. It's a "straw that breaks the camel's back" situation, so it's better to be safe than sorry. It is important to note, also, that airbags rely on having an intact windshield in order to deploy properly. If it isn't fully up to strength, they may just blow out the glass and not protect the people inside at all.
The length of a crack may not come into the decision. If it's in the outer laminate only, if it's in a position that doesn't impair the driver's vision, and if it doesn't appear to be lengthening, then it may be safe to leave it for a while. But it must be kept under observation. Cracks can lengthen with startling rapidity; on one of my cars a crack became 18" longer over the course of about eight hours!