I'm doing 8 weeks of my current conditioning/endurance phase But the rest i've found around 6 seems to be ok for me personally. I do less for power, usually around 4 weeks as it can be pretty intensive
In regards to the on going debate about stretching: I feel hella stiff if i don't stretch before exercise. Mainly if i go running or throw the pigskin around.
Dynamic stretching is fine and i feel it loosens me up a lot better than static anyway. Before i knew better i used to statically stretch before a workout and would still feel tight, especially around the groin. Now i do forwards, backwards and side leg swings amongst others and feel so much better. Football coaches are notoriously bad for being behind the times with their training methods, yet they still wonder why so many suffer from torn hamstrings etc!