Notification parsing times

I present "notifications", from time to time (as do most apps).
These may be error messages, reminders, etc.

In many cases, I do NOT want the user to have to acknowledge
the notification for it to be dismissed -- this adds another
step to the UI that may not be convenient for the user.

Assume device is tied to a particular user (at some point in
time -- i.e., can be "customized" for their needs)...

How to determine the duration of such "notification displays"?
It's impractical to allow a time to be specified for each
specific display.  Adopting a single timeout means ALL
notifications must be parsed in that interval, regardless of
how familiar/novel any particular one may be to the user.

Perhaps a set of timeouts -- short, medium, long -- that the
developer assigns to particular notifications that the user
can then tweek (i.e., "make all 'short' notifications take
3 seconds")?

[I've been examining various apps/devices to get a feel for
how long they interrupt the normal flow of operation for
these events.  All seem to favor terse messages that the
user can recognize quickly and mentally dismiss.  The idea
of presenting more detailed context/assistance would make
such short timeouts impractical]

Or, notice how long it takes for user to move past a particular
notification (they self clear when the user continues whatever
he was doing DESPITE their presence) and use that to "learn"
the timeouts appropriate for each (notification,user) tuple?

[User can always resort to an "ACKNOWLEDGE" to dismiss the
notification without having to recall what he was doing
when "distracted" by the notification.  Doing this each
time a notification appears is undesirable]

Dealing with a device that *isn't* tied to a particular
user gets a bit more complicated...