I recently read Pharos, and in it, the loyalists essentially overcharge a Necron relic in order to defeat the traitors trying to control it. This sends out an energy pulse into the universe, a pulse that's detected by the Hive Mind. It then turns its mighty attention in the direction of the Milky Way galaxy.
So, this happens during the Heresy, putting it between 009.M31 and 014.M31.
The first Hive Fleet enters our galaxy around 740.M41. That puts it at about 10,700 years it takes for them coming through the warp from across the intergalactic void.
So, on to a curiosity. On a good day, it takes about 7-9 months to cross the Imperium's northern half (about 500,000,000,000,000,000 km / 50,000 light years / 15.3 kpc). Presuming the Tyranids are always having a good day in the warp, can we extrapolate the distance, and approximate which galaxy the devils came from? I think so!
So let's go. Let's presume the following:
- 8 month travel time = 15.3 kpc
To reduce it to a 6 month value, we have 11.5 kpc. To put it all together, that's a distance of 23 kpc per year, times 10,700 years, equals a grand total of 246.1 kpc, or 802,286 light years.
There is one known galaxy around that distance away from us: the Leo I galaxy, a dwarf spheroidal galaxy. It's not nearly the closest galaxy to us; it's the 17th closest galaxy to us. Makes you wonder about the rest and what secrets they hold...