The spacing in time is speculated based on numerous item descriptions. The description of the Sunlight Parma or Sun Sword can be used as a good base for the speculation as both imply that the covenant and it's members are very old and nearly forgotten. As a matter of fact the Lightning Spear miracle itself references a large span of time, it's description reads: "A miracle that launches a spear of lightning. Said to be the legacy of an ancient clan whose leader was revered as the God of Sun. The name of the clan has been lost to time, but the gross incandescence of our magnificent father shall never wane.". Lost to time, kinda makes you wonder. Anyway I'm in agreement about excluding Dark Souls II lore/story, there's really no need to involve it.