For anyone having the same problem as the OP, ending up here for an answer: These days you can find it in <scheduled-task> elements in the relevant subscription's XML file.
Thank You, that's what I was looking for. However I'm more comfortable with the following plesk command (probably this option didn't exist back in 2008).
plesk bin server_pref -u -db-user-length 10
More about setting server_pref:
server_pref: Interface and System Preferences