As far as I know that’s as deep as wordpress can go, you will have to use php to get it into weeks/months/etc.
You have 2 options:
Use human_time_diff and create a function that just calculates the differences(pretty easy to figure out 7 days = 1 week, etc). I would honestly not use human_time_diff though and use strtotime() with this method instead.
The other option is to use the DateTime and DateInterval objects in php 5.3 +.
You can see an example here ( second answer), https://stackoverflow.com/questions/676824/how-to-calculate-the-difference-between-two-dates-using-php