I don't think you can calculate it like that.
This is the way how I THINK it goes:
player has x week injury. Next dayly update the system checks for injuryadaption: the chance the number goes down = formula which holds a random factor and the days since change.
each day the change it drops is getting bigger, until it drops and then you start over until you reach 0.
BB probably has programmed it so that it is likely it will get to 0 on day 7, but you can't tell.
An example:
a 3 week injury. on day 1 chances are practically 0 he will heal, same on days 2 and 3, on day 4 chances get bigger but still small, on day 5 and day 6 chances keep increasing, but still no healing. day 7 chances are good he heals a week, but you are unlucky and the random factor makes him stay at the same amount. You are even more unlucky and day 8 still no change. Chances are pretty high he will heal on day 9, but you won the lotery in the wrong way, no healing. On day 10 you get lucky (heh, what is called lucky, you should be VERY unfortunate not to see him healed, but I think it's still possible).
If this repeats itself, you only get him back after 30 days.
The other scenario is that you are so lucky he heals every 5 days and you get him back after 15 days.
Most cases will happen in between, but nobody can tell what happens if you get REALY unlucky. I don't know if there even is a limit on the max of days a "week" can hold, or that chances of healing just keep going up but never reach 100%, and you might get an exceptional case where 1 week lasts like 30 days... (however I don't think that's the case, logically speaking they will have limited it at some number, but nobody knows which)
So theorethically and averagely speaking you can say yes: a 3 week injury will last between x and y days. But in reality, you can never be sure.
They are not your friends; they dispise you. I am the only one you can count on. Trust me.