Suppose we want to find the time needed to write down all possible combinations of 58 characters.
With a string of size n the possible combinations are 58^n.
We pick random characters every time we write down a string. If I write 1000 strings /second how much time will I need to write 50% of the possible strings?
How can we find the time for any percentage?
If you want the expected time needed to write down some fraction of the possible strings, you can use the same argument as in the coupon collector's problem with $58^n$ coupons by adding the expected time to see a new coupon until you reach whatever fraction of the coupons you are aiming for.