The teaching is really to be 'mindful' of Buddha, not praying. It is really no different than other 'mindfulness' meditation used in other traditions, the only difference is what you are been mindful of. The Texts specify that a practitioner needs to have 'one-point-concentration' of Buddha for 7 days, (i.e. no other distracting thought and probably not even getting up to use the restroom or to eat.) in order to be reborn Pure Land.