The backups will usually be the first players who will substitute for the starters. if your players don't have much stamina, the backups and starters might play about the same amount. If the starter is better than the backup and he has OK stamina, he might play 40+ minutes. The reserves usually don't play much, unless you have an injury or foul troubles.
A player can be a starter at one position, and a backup at another, and a reserve at a 3rd, or a backup at two positions.