Application Time - As per NFPA 11 (Refer to table 5.8.1.2 Minimum Application Rates and Discharge Times for Nondiked Spill Fire Protection Using Portable Foam Nozzles or Monitors)
The SVFD Quick Reference Field Guide is based on an application time of 15 minutes.
15 minutes is not a magical time where the incident is problem solved.
The 15 minute period includes knockdown + more application time to improve foam blanket quality, fuel cooling etc.). After flames have dissapeared, the fuel may still be emitting vapors which can reignite. The foam & film are suppressing the vapors but there will be foam breakdown and effects from foam disturbance.

Foam disturbance - 
All of the factors that effect foam breakdown are essentially going to cuase foam or fuel disturbance. There are some additional considerations which may help you decide to increase the application rate or the application time.
The basic advice: Do not robotically apply an app rate for 15 minutes. Take all factors into account and adjust accordingly. "Over estimating is preferred to underestimating".

Wind can easily effect a raindown application method or can blow foam away from the fuel's surface.

Jagged material, vertical surfaces, and hillsides will effect Application rates.

The "Film" in AFFF does not form the same on vertical or rough surfaces such as grass, gravel, or other material.
If the fuel is flowing, so is your foam. Finished foam product may be carried away as new fuel is leaking from a container.