I've answered my own question by running an experiment:

1. Put 990 in Standby.
2. Measured time before each of my two 2200s went into standby.

The interesting thing is that the first — the top one in the stack — went into Standby after 9 minutes, but the second — the bottom one in the stack — did not enter Standby until after 14 minutes had passed.

The 2200s are on their own shelf and stacked atop each other.