just this '南無觀世音菩薩' ? how many times? =)
You can chant it as many times as possible. It's up to you.
Minimum should be 7 times a day. Normal practice for buddhist is
108 times a day (representing 108 sufferings are gone when you
chant it). Buddhist usually uses the buddhist beads 佛珠 (108 beads)
when doing chanting. Buddhist beads are usually used for counting
no. of chants. There are also buddhist beads of 21 (worn on wrist),
54 beads.
I usually chant 南無觀世音菩薩 whenever I'm in the MRT going to work or
going back to home after work using my 21 buddhist beads (which I
wear on my wrist).
There are other versions of chanting for Guanyin (depending on
your own practice):
南無大悲觀世音菩薩 (Namo Dabei Guanshiyin Pusa)
南無救苦救難觀世音菩薩 (Namo Jiuku Jiunan Guanshiyin Pusa)
Note that "南無" (namo) is a
sanskrit term that means "pay homage" (敬仰,受敬,依賴). It usually
appears in front of Buddha or Bodhisattva 's name when one is
chanting Buddha/Bodhisattva's name.