Usually is concurrent. How long can reference take man... Its just a call or sometimes email. It's the least of your worries. Be more worried about clearance such as financial embarrassment or criminal record in the family.

I think you are trying to predict how long more they may take. Give your mind a break, do something to take your mind off, this is something beyond your control or influence. Chill buddy. Keep applying jobs, assume the worst. I know it's easy to say hard to do... But really there's nothing u can do.