three important things in this case:
1- will the match be a blowout?
If yes, expect around 20 mins for your backup.
2- is there a great skill difference between your starter and your backup?
If yes, i myself have experienced 44-4 minute distributions between players even when strictly following depth chart. Because my backup was way too worse than my starter.
3- what is the stamina level of your starter?
lower stamina for your starter will yield more minutes for your backup.
To sum up, if you have incredible skill difference between the players and the starter has 8-9 stamina, your trainee may not get 8 minutes. In every other case, set him as backup.