I've been wondering about the stretching speed of heroes with elongation powers. How fast and far can someone like Mr. Fantastic stretch in a second? I am looking for an answer based on canon.
His transformation is instantaneous. He can take any shape in just a matter of seconds. Although he wouldn't compare with the light speeds of Thor or Mach speeds of Iron Man, usually his reaction is in seconds, fast enough to save someone from a split-second accident.
Quote from the Powers and Abilities section on the Marvel wiki:
Mister Fantastic can alter his form in a matter of seconds, often much less (depending on the complexity of the shape), and revert to his normal humanoid shape within a similar time. The greater the distance he stretches or the more extended the size of the object he becomes, the weaker his overall strength becomes. Mister Fantastic's transformation to a malleable state is reflexive and nearly instantaneous: if he was at his normal form and taken unaware by machine gun fire, his body would still absorb the bullets' impact through radical deformation.