The flight
Thrown, slowed by its drag, falling.
The flare leaves the dispenser at the cartridge's ejection speed, 30.3 m/s (US patent 5,400,712 measured 99.5 ft/s), on top of the aircraft's velocity through the air. From then on two forces act on it: its drag and its weight.
It is a small dense block, tumbling. A tumbling convex body shows, on average, a quarter of its surface to the flow (Cauchy's formula), and a block's drag coefficient is a cube's, about 1.05 (Hoerner, Fluid-Dynamic Drag, 1965). At 250 m/s near sea level that slows it at about 770 m/s², nearly 80 g: it is far behind the aircraft within a second. Once slowed, it falls at the 28 m/s its drag allows at sea level, a little more higher up. As it burns, it gets lighter for its size, and slows faster.
The flares are laid in the air, not in the aircraft's frame: each moves at its own velocity less the aircraft's, so a turning aircraft leaves them where they were let go. The air's density is the standard atmosphere's at the altitude, or the shared flight's.
The steps are at most 5 ms, the drag taken implicitly, so a long frame or a slow tab flies them the same.