It can take up to 10 working days.
Why can't it be used, what errors are displayed or given ?
A license is active from when you purchase it not from when you first use it.
If you purchased a license for avast 7 that would be well over a year ago as avast is on version 11 now. Whilst you could still purchase a license if you had avast 7 it would be somewhat unusual and it may not work on a very old avast version.