Today J&J released an 8-month update study in a small subset of trial participants (8 people), including information on the delta variant. They chose an older subset ranging in age from 47-91, in order to be sure that it's not wearing off in the older population
Like all the other vaccines, antibodies elicited by J&J's vaccine have somewhat reduced activity in neutralizing the delta variant in a pseudovirus assay, although not as much as for the beta variant. Two of the eight have lost neutralizing activity against beta and P.1 (the Brazil variant that is also common in the US). All eight still have neutralizing activity against delta, although it's pretty low for those two.
These results are about what I would expect. The data I've collected so far show that if anything, the antibodies from the J&J vaccine are going up slightly over a period of months, and J&J also published some data to this effect earlier. This also appears to be happening with a single dose of the RNA vaccines. It's when you give two shots of vaccine that you get the high antibody levels that then start to decline. This means that at 8 months, the antibody gap between RNA and J&J is smaller than it was in the first few months.
Link to study : https://www.biorxiv.org/content/10.1101/2021.07.01.450707v1.full.pdf