found a definition I hadn’t seen yet.
siddhi : the word by itself means supernatural abilities given to the masters of eastern religions. Each of them has their own list
mantra siddhi : this means that you have achieved perfection in pronunciation of the mantra. It also means that you have achieved perfection (as in completeness) in receiving all the benefit that you can from a particular mantra. There is no exact number for achieving mantra siddhi, but as a guide count the number of syllables in the mantra and multiply by 1,000. This will give you an estimate for how many time to do the mantra (those hindu mantras are 20-30 syllables long 
)
most of the mantras on the forum are 6-8 syllables
adding the new @DarkestKnight mantras to the list: Angelic Mantras