In Jamgon Kongtrul's Knowing One Liberates All
, He says it usually takes six months of recitation retreat (when going by time) to accomplish a given Deity. However, each sadhana typically spells out how many mantra (when going by number) one should expect to recite before achieving accomplishment. In most of the sadhanas I am familiar with, the recitations are divided into three stages: nyen-pa (approach [familiarization]), drub-pa (accomplishment), and lay-jor (activity), with each stage/mantra being said a certain number of times. A general rule for nyen-pa is to say the root mantra 100,000 times for each syllable with possibly another 100,000 for good measure, i.e.
, to make up any deficiencies. However, in the Paltrul sung-gyud (Paltrul Rinpoche's lineage of Longchen Nyingthig [as, for instance, carried on by Kyabje Chatral Rinpoche]), one typically does 11% over and beyond what the text stipulates. So, if the text says 100,000, the practitioner does 111,111. I agree with Dechen Norbu that it usually takes serious time and effort doing the root mantra before being able to make the activity mantra really work. I have also heard one Lama say (on several occasions) that reciting more than just a few of the activity mantra before having done the requisite nyen-pa and drub-pa is a breach of samaya that can shorten the Teacher's life. If that is true, then I think it goes without saying that it would also be bad for/dangerous to the sadhaka that did so.