Well. It varies...
First of all, on *IX all filesystems usually have a reserved space, accessible only to root; nowadays it is 5% of the filesystem size (by default), but can be larger or smaller. If you are running as the superuser, then there is no reserve.
So, whatever is your filesystem size, you can actually use up to (100 - reserved space).
There is no fixed response to your question: it depends on how you use the filesystem. For normal usage (i.e., a desktop/laptop with email, browser, music, videos), a probable safe bet is around 60-80%. If the disk contents is volatile -- meaning lots of changes, large files being created/deleted, etc -- then you can expect degradation as the filesystem gets filled up. Now... of old, a good initial bet was "be worried if it is 80% full"; but with the current crop of extremely large drives, it does not make a lot of sense anymore -- 20% free space off 1T is still 200G! -- and this is one of the reasons the reserved space is now only ~ 5%.
But... rule of thumb for generic use... I would say tops at 70%, then start considering an upgrade/additional drive ;-)
If, OTOH, you are just adding files one after the other, never (re)moving them, then you can safely use all space available with no impact.
And yes, there can be fragmentation, which can cause degradation on read/write times.