10 or so minutes post-workout. I follow a fixed set of stretches/poses and usually intend to hold them for longer than I do. I should be stretching upwards of 15 minutes, so I guess I need to concentrate on holding them longer.
I occasionally stretch at other times throughout the day. If I'm feeling tight/sore, or have been doing any other form of activity (eg. I might stretch after taking the dog for a walk or doing household chores), I sometimes stretch. Not often enough though.
Pre-running I do mobility drills. I use warm-up sets during weights.
I need to improve my flexibility for injury prevention purposes, so I really should try to stretch more.