It's a bit like trying to predict the exchange rate.......
There was a something on Skyscanner which said that their research suggested the cheapest time to book a flight was about 7 weeks prior to departure.
By coincidence, I booked an outbound flight ( July 28th ) at the beginning of June and since then it has gone up £100.00.
I booked the return flight about a fortnight later and it has gone down £20.00 today.
If you are flexible on dates then you might be lucky and get a bargain. If your dates are fixed then it might not be worth the gamble.
Booking the round trip as two separate bookings ( out and back ) has paid off for me in the past. It always seems to be the return flights that drop in price a little a few weeks before , but as you get close to the actual date I have only ever seen all flights go higher and higher....... no doubt some one will now say they have seen different.