Reading the Timing Charts
Here are three different timing charts:
On the first one it shows drawing 34 is half way between drawings 33 and 35. This movement of three drawings would take 6 frames (if shot on twos) and would take 1/4 of a second to move from drawing 33 to 35. That's pretty fast.
The middle timing chart has 3 inbetweens between drawings 36 and 40. 37 is half way, just like 34 was. 28 is half way between 37 and 40 and 39 is half way between 38 and 40. You would do inbetween 37 first, then 38 and then 39. This timing chart is indicating a slow out of the action. This movement of 5 drawings would take 10 frames (if shot on twos) and would take almost 1/2 a second to complete from drawing 36 to 40. Thar's just a bit slower than the previous action.
The final timing chart has 5 inbetweens total. 46 is the half way position just like 34 and 35 were. 41 and 43 are called "quarters" because they are 1/4 the distance between 40 and 46. They are physically 1/2 the distance between 40 and 42, and 42 and 46. You would not do inbetween 41 first because it is difficult calculating the 1/4 distance between two lines, You would do the half way inbetween first which is 42. Then you would do 41 and then 43. After that you can do 44, half way between 43 and 46. Finally you would do 45 between 44 and 46. This movement of 7 drawings would take 14 frames or just over 1/2 a second.
All of these are still pretty fast at under 1 second to move from key to key, but it all depends on what the character is doing and if the timing is appropriate.