My older son (will be 4 in two weeks) has been in OT for severe sensory issues for the last 2 years. I know he DOES have these problems but I don't think in his case the therapy is working. But he enjoys it and the insurance covers it so I just look at it as being nice that someone else other than me works with him one on one. We've had two great therapists over the years, so the lack of progress isn't due to them. It's more so that his issues come and go. He outgrows something and something else pops up.
Now, as to your question, there are times when I question if he really needs the therapy ... it's the periods of times when he's doing better (between problems coming and going) ... I also have friends (and even my husband) who question why we do this and they think he's just a normal slightly ADHD little boy BUT these friends that question it themselves have kids who in my mind need some serious therapy (one of them finally got her kids evaluated and one is in special ed preschool now and the other is getting therapies through early intervention) so I think it's people who are in denial about their own kids who are the ones to tell you your child is fine. Just follow your gut. If you feel like it's not worth it, forget it. If you feel it's helping, keep going ... We now work with two therapists ... one is OT (the one my son loves!) and the other is a physical therapist though she does mainly OT work with him and I can see it's not working too well with her. She's older, the owner of the place, with tons of experience ... but I just don't see her being a good match for my son so comes fall, I'll probably come with some good excuse why keep just the one OT and not the PT. We started with the second one as she was going to try some other methods (cranial-sacral and related "touch" therapies that the OT is not certified for) but I see these methods backfiring and not working for him. They work for most kids with sensory issues but in his case they make him more irritable and now even angry. ... so, again, I'll be following my gut and if things don't turn around in a few weeks, we'll be getting less.