Dear Mike,
I'm quite surprised that you're saying that you "changed your update settings (both program and database) from manual to ask-when-available" as the defaults really are:
- virus database (VPS) update - automatic
- program update - ask-when-available.
That's probably the optimal setting for most of the users.
However, to answer your questions, unfortunately it's currently not possible to customize the time period that the popup stays up (15 seconds) but the fact is that reappers every time it tries to update, e.g. every 4 hours in case of the VPS and every 12 hours in case of the program. I.e., even if you miss it, it will soon appear again. AND, if you want, you can try to manually start the update by right-clicking the avast tray icon and selecting the appropriate option.
It's worth noting that program update always includes the VPS update (i.e. if both new program and VPS updates are available, and you'd like to apply both, you only need to do the program update, the VPS will by updates automatically).
Hope this helps,
Vlk