You can right click on the avast tray icon, avast! Shields control, Disable for X time, the avast self-defence module will ask if you want to continue, answer Yes. See image, click to expand.
Whilst I would suggest as short a period of time as possible, there is no estimate of how long combofix will take. So you might want to go for 1 hour or until the computer is restarted (combofix could well require a reboot). The main thing to remember is to either restart avast shields or reboot to ensure they are started again so you aren't unprotected.