My personal experience has been 1000 and 1500 seem to be the most common. if you want a third it's a toss up between 1250 and 1750 depending on whether you play large or smaller games. I don't know where people get some odd numbers like 1850.
"There is a cancer eating at the Imperium. With each decade it advances deeper, leaving drained, dead worlds in its wake. This horror, this abomination, has though and purpose which functions on an unimaginable, galactic scale and all we can do is try to stop the swarms of bio-engineered monsters it unleashes upon us by instinct. We have given the horror a name to salve our fears; we call it the Tyranid race, but if it is aware of us at all, it must know us only as Prey."