The length of a cq call should be kept flexible. The length of a call should depend on the density of stations on the band.
Short calls are appropriate on a wide open band with many stations, long calls are the choice for a half closed band or VHF and up. The exact frequency of choice depends on what you want. If you feel confident with operating techniques and if you are equipped for DX the lowest frequencies of the CW bands are your choice. If you prefer to chat the upper band edge is the appropriate choice. QRP operators choose the QRP calling frequencies or a little higher, many CW clubs do also have preferred frequencies.
The format given by the example is workable in most circumstances. When in doubt, it is always better to tend to short calls and to repeat them more often. It is kind of embarrassing if you call too long and get several callers at a time.
Common mistakes
too many cq and to few id's. Consider that there are many listeners who are hunting for particular stations. They scan the bands and stop at any cq call. They get impatient if the endless cq sequence is not enriched by a call sign.
Avoid to be impatient after the cq call. Listen with sufficient bandwidth and/or check neighboring frequencies. Any ham feels bad if he or she answers a cq call and the response is a continued cq call.
Do not use 'kn' at the end. Finish the cq call by 'K' or by 'PSE K' (or by 'CQ K')
Be cautious with directed cq calls such as 'CQ DX CQ DX' because such calls exclude others from a QSO with you. Of course, directed calls are legitimate and if you hear one, please respect the wish of the caller.