First off, we must bear it in mind that we are talking about the use and the sense of the determiner some in front of time, hours, months, years, etc.
According to The Free Dictionary, you use "some" with "time" and words such as hours, months, years, etc. to refer to a fairly long time. For examples:
You will not be able to drive for some time after the operation.
We've been here for some hours now.
On the other hand, when you refer to a short period of time, you don't use some. Instead, you say a short time and use "a few" in front of hours, days, months, years, etc. For examples:
Her mother died only a short time later.
You'll be feeling better in a few days.
In addition, you van use the phrases "some little time", " some few hours, months, years", etc. to mean a short petiod of time. For examples:
Her mother died only some little time later.
You'll be feeling better in some few days.
However, if you want to refer to an unidentified or unknown time, you can use "some" before time. For examples
I saw him some time/sometime last summer.
We'll see some some time again, I am sure.
So whether some time means an identified time or a considerable/long time depends on the context.
As for "some years" in the sentence "We have known each other for some years" means a long time; a lot of years.