Probably the same time a Mormon thinks is proper to tell you the truth.
And what would that truth be?
Hmm so Mormons need to be taught “the truth” by others claiming to know it.
There are about a zillion different denominations, with a zillion different teachings all calling themselves Christians, all using the Bible and all claiming to have the truth.
But Mormons can’t possibly be Christians but all the rest are with their different teachings are.
By definition, whenever two true believers of different religions meet, the sum of beliefs of each person will be The Truth to one person and Heresy or Paganism to the other.