I'm playing my second campaign now (H/VH - Turks) and I'm wondering when these dreaded Mongols & Timurids show up? Does it happen at a set turn, specific year, or is there another trigger?

My first campagin as the English I expanded too quickly and finished before I ever saw either (I assume that there'd be an annoucement similar to the BI Horde annoucement so I would know even if I didn't see them), but also before I got any gunpowder units, or heard about the new world.

I'm expanding much slower this game so I get to do all this fun stuff, and to make things interesting for the campagin I'm going to try to hold onto everything from asia minor over to Tblisi and down to Bagdhad during the invasion. All I'd like is a hint about when to expect these eastern guests so I can plan to have some semblance of an army or two in the east before they arrive, right now most of my troops are towards my western and southern borders.