We started when ds was 3.6, just before he was diagnosed. I had met someone locally who was doing it with her ds and even though ds hadn't been dx I a) knew he would be, and b) even if he wasn't I thought he needed some help.
It was really helpful seeing another programme in action - maybe you could try and find other families locally?
We did VB, which is rather more Natural Environment Teaching than 'traditional' ABA which can be more table-top based, though in reality there's not much difference. We chose VB as ds is pretty high-functioning and the main focus of his programme was teaching play and social interaction (he had many copying skills already).
The main problems were play skills, social interaction and social communication - he had some functional speech but it was limited, and no conversation, and his interaction with other children and unfamiliar adults was extremely limited. He had no imaginative play at all, and his play was repetitive and limited. He had rigid behaviour and tantrums as a result of either not being able to communicate, or being forced into situations which he wasn't ready for.
We started off with 4 days a week (24 hours), then down to 18 hours, and 2 of the days were at nursery where his tutor shadowed him. He is now over 6 and a half (7 in April) and we currently don't have any formal teaching since our last tutor left in November on maternity.
It achieved everything we thought possible, and more. It's only now actually that I can see how impaired he was at the time (compared to now), even though he was always high-functioning. He is hugely interactive and engaged, is able to communicate effectively, plays really well, has genuine friendships, is at mainstream school and top of his class with no academic support needed, and his behaviour is fantastic. He is also now motivated to do things and try new things simply because he can see that they're fun and now has the skills to do them - eg before he would not attempt social interaction because he simply wasn't equipped with the skills he needed. I remember asking our consultant very early on how it would be possible to teach a child to want to interact as I coulnd't see how you could, and the consultant said it was like learning to play the piano - you just need to be taught the right skills. And honestly, it happened before our eyes.
He does still need help with things like understanding other's intentions, realisng the impact of his behaviour on others or their behaviour towards him, and he can still get obsessional about things (eg measurements of time/units/size) and can launch into conversation without caring or noticing if the other person is interested, but these are things we can continue to help him with. His pragmatic language also needs work, as he does take things at face value, so can't always work out something by inference.
But he was genuinely transformed within 6 months-a year of starting. And most importantly, he is also 100% happier than before - his world has opened up and as his skills have increased, so has his confidence to try things, so he is now able to make decisions for himself to do things which he will enjoy, whether that's activities (he does loads of clubs etc) or just having a playdate at a friend's house.
I know not everyone has such dramatic results as us, depending on the severity of the ASD I guess, and we were also lucky that ds didn't have too many severe behaviours that might really make it harder to teach (eg stimming, language, sensory) but honestly no-one I know who has done ABA, both in RL and online, has regretted it for a second.
The setting up of a prgramme, finding good tutors, and of course the cost is difficult, but once you have the programme set up it runs itself really. Also getting everyone else on board (DH, family etc) is crucial as you all need to be consistent.
If you have a consultant or programme provider already maybe they could find a local family who would let you & DH sit in on a few sessions? Like you my DP wasn't convinced at all to start with but it was obvious simply by the results after just a few weeks that it was the best thing to do.
HTH.