ylchen,

The speed at which the supporting heel releases depends on a number of factors, such as the speed at which you want to rise, if at all. But on a basic, level forward walk, "gradually and throughout" is probably the best language I can use, without getting too technical.

Think about the speed at which the back heel lowers on a back walk: It begins lowering at the moment the foot stops moving back and weight begins to transfer onto it. It lowers gradually an continually throughout, making contact with the floor at roughly the same time as the body arrives vertically over the foot, which also happens to be roughly the same moment the moving foot passes it. (People have varying opinions about the fine-tuning details, but we'll call that the "happy medium").

Now take that action and play it in reverse. Voila! The forward walk.

From the moment you begin to move the body forward of the standing foot, the heel begins to rise right away. It rises gradually and continually as you move away from it, so that when the back leg reaches its fullest extension, the heel is at its highest point. From that point forward, the heel will begin to lower again as the moving foot moves towards the standing foot.

As an aside, some will argue that the heel should continue to rise for a brief moment beyond the point of mid-stride, so that the foot can reach a full point to the toe (At mid-stride, with weight still on the foot it has only really reached the ball). This might be something to strive for at the most advanced level, but I advise againt it from the standpoint of teaching the average dancer. 99% of students I see make the opposite mistake and roll the foot too far over and for too long, resulting in a slight forward pitch of the body into the next step. There's also an odd tendency for students who focus on pointing the back foot to turn it out, as in Latin, resulting in horrible tracking on the next step. So I have found myself needing to teach most students to start the next heel lowering immediately after mid-stride, focusing less on pointing and more on the "stroking" action of the free foot. As a student, you know you're ready for a more pointed foot when you can accomplish it without sacrificing the tracking and stroking actions of the free foot, or the leg timing or posture.

But I digress. You were asking about the speed of the lifting of the heel. Take a look at the image sequences. You'll notice that in both instances, the heel is lifted gradually and throughout. Victor's heel seems to rise slightly later than Timothy's heel does, but they are very similar. The difference is likely based in the fact that one is dancing slow Foxtrot, the other Quickstep. In any event, they are both very well timed. Strive for something in that ballpark, and you should be very pleased with the results.

Regards,
Jonathan