Once a composition is started the duration of an aksharam has to be kept the same. However, the music may have one note per aksharam, or 2 notes or 4 notes and so on thereby changing the duration of notes and making the music appear faster. This aspect of Carnatic Music is called Kaalam. For lessons up to the Geetham the first kaalam is sung with one note or swaram per aksharam, the second kaalam with 2 notes per aksharam, the third with 4 notes and so on. (A long note of double duration is counted as 2 notes.) You can listen to pulses of 3 kaalams in Aadhi thaalam. Note that the kriya or thaalam action is at the same speed for all the kaalams. Also listen to notes of Maayaamaalavagowla in 2 kaalams.
Varnams are written with 4 notes per aksharam and often when Varnam is sung in 2 kaalams, the 'first' kaalam is with 4 notes and 'second' kaalam is with 8 notes per aksharam. In writing notation for Kritis no uniform practice is followed. You have to count the number of notes and find number of notes per aksharam.
Although no absolute duration for aksharams is prescribed normally they take between .5 and 1.2 seconds. When aksharams are of longer duration the beats, counts and waves may be repeated once to enable accuracy in time keeping. This is called 'Two Kal(l)ai Chowkam', while the normal method is called 'one kalai'. Aadhi thaalam with 2 kalai may appear to have 16 aksharams as each count is repeated once i.e. 2 beats, 6 counts, 2 beats, 2 waves, 2 beats and 2 waves. Listen to 2 kalai in Aadhi thaalam-the repetition of action is indicated by change in color of the arrow which remains in the same position. The example (Vathapi Ganapathim) in page 2 is in one kalai. 4 kalai chowkam (very slow) is sometimes used in Pallavis.