If you are using something more complex, say Gāyatrī Mantra……
“The number of times you say OM on inhalation, holding the breath,
and exhalation is influenced by the length of the breath.
We cannot fix the number of recitations on the basis of the Praṇava itself.
We can only fix it on the basis of a person’s capacity of breath.
If you are simply using OM, it can go with almost any ratio.
If you are using something more complex, say Gāyatrī Mantra,
it is very long and has different structures so there are regulations on
how many times you say it when you inhale, hold the breath, and exhale,
and in what part of the Mantra you can break, etc.”
– TKV Desikachar Religiousness in Yoga
‘Various Approaches to Yoga’
Chapter Seventeen Page 238-239