> Either way, it wont suddenly turn "on" overnight. These things ramp up and down instantly on cosmological scales, which means many thousands/millions of years in our world.

Some supermassive black holes have been caught briefly turning on, over timescales on the order of weeks-months. Tidal disruption events, where a star wanders too close to a SMBH, can result in luminous accretion with significant accretion rates (e.g., Figure 3 of [0]). As the material of the star falls into the SMBH or is blown away, the luminosity drops off, with a predicted decay of t^(-5/3).

[0] https://physicstoday.scitation.org/doi/full/10.1063/PT.3.238...