If a sequence avoids three term arithmetic progressions it is less than ((log log n)^4)N/log N according to "A Quantitative improvement for Roths’S Theorem
On Arithmetic Progressions" which is available at
http://arxiv.org/pdf/1405.5800v2.pdf

The above preprint also claims that the result of "On Roth’s Theorem On Progressions" availabe on arxiv here: http://arxiv.org/pdf/1405.5800v2.pdf
is in error in claiming O(((log log n)^5 N/log N) and that it should be
O(((log log n)^6 N/log N).

So the upper bound even for three term arithmetic progressions is at most a factor of log log n to the fourth power better than N/log N if the preprint in the first paragraph is correct.