If you mean how long as an SO then there is not really a average time because only a handful of SOs have been given freighter FO positions. It could be right away or could never happen. All depends on how the initial course goes and how the line checks etc. The longer you wait as an SO the more chances you will pooch something and then you will not be offered an FO slot. Better off to try and ask before you start your course. A polite phone call to recruitment might steer you friend in the right way.
If you meant FO to Captain well almost the same story !!
Cheers
FG