I would say it depends on what you want to convey to the reader.
The second option definately puts more emphasis on the long timespan by starting of the sentence with ''For long, ...''. So if it is really important to you or the reader that it is not only known but known for an extended period of time (maybe talking about ancient knowledge or something), this is probably the better choice. In that case, however, you might also want to consider being more precise conceirning the timespan:
Since ancient times/ the middle-ages, ...
For centuries/ages/, ...
If it is just about the knowledge and the ''long time''-part is merely some additional but minor information for the reader, ''It has been known for a long time ...'' might be a more adequate choice.
But really, the differences are subtle and both options could be just fine in your case.
There are similar cases where the word order defines emphasis, for example:
In chapter 2, we have shown that ...
vs
We have shown in chapter 2 that ...