It's based on gut feeling mostly, and understanding the instance mechanics. If the tank has to position the enemies then he'll need more time for aggro than if he tanks on the entry-point, if there are adds he may have to save some aggro-builders / force-taunts for them, and so on. And some instances have special aggro-mechanics on top of that.
As a rule of thumb I'll give the tank 10s for aggro-building (after positioning) when on champ/rk, as a hunter I'd probably wait a little longer their DPS doesn't take as long to build up.
But again, this can vary based on instance. Also depends a bit on what skills the tanks starts with, some people prefer to start with challenge+engage and let the DPS people go wild within the first few seconds already to get a solid aggro-lead.
Used to play: 85 Champ / Captain / Runekeeper / Guardian, Guild Master of everything but cooking.
Playing now: Hellcat / King Tiger / GW Panther / IS / KV-5 / M4 Sherman and more