It could be your husband is lying, why won't he take you to an appointment? Does he talk to you about his therapy after he goes, or just when he is defending the way he is treating you? Things to think about anyway.
We had three therapist telling us a full detailed timeline was a bad idea. I didn't listen, he eventually didn't listen. I had to tell him I was leaving the next night if he didn't give me at least the start of it by then. It was not a bluff. Most therapists think timelines are too damaging to the BS. Mind movies and flashbacks have more ammo. I know I would have been trapped in the questions forever. I was also trapped in rage until he gave me the full truth. Just today our MC was telling me how amazed he is that the full timeline is working so well for me. That is in part because my WS is doing the work, to fully be there for me when I get hit by triggers. There are a million triggers.
That being said my WS had a therapist that was either a cheater, or wanted to be. I know all of this is true at this point. WS started seeing jackass therapist (JAT) while he was actively paying women to "date" him. JAT told him it was reasonable, never told him to talk to me, he never asked him what he wanted out of his life. WS was in full denial these were prostitutes, JAT never even gently had him examine who these women were, and yes JAT knew WS was paying them. He never pointed out that WS was putting my health at risk. WS spiraled into worst craziness, saw several of them during the same period, was talking about not being sure about what he was doing, he got reassurance from JAT, more it was reasonable, among others. When WS figured out that his wife of 20 years (me) was the one he wanted to share his life with, then JAT, told him to talk to the one hooker WS was still paying, horrible advice, this woman was a con artist! JAT knew that the husband of one of them (long story) had sent a letter to me, at my physical address, telling me about the one affair, WS activity on a sugar baby site and that WS had stolen the letter. JAT still told WS not to tell me anything to spare me the pain.
After I got a registered letter on Dday from the husband, JAT continued to tell WS not to tell me anything. When WS told JAT about my rage, all he told him was "that sounds tough, are you sure you want to put yourself through that?" Even though WS had been clear he wanted to save our marriage. I finally figured out how horrible JAT was when WS started physically hurting himself. I was concerned, and asked what JAT told him. I was thinking there would be coping tools or something, he was asked again if he wanted to put himself through it. He finally change therapist, and things got better.
I will say, even JAT offered to see me, before I knew about the cheating. I didn't go because I didn't want to insert myself into WS's therapy, and I am disabled from pain, so there would have had to be a huge reason for me to go. My point is there are horrible, damaging therapists out there. If your WS has one, that could compound your problems. This does not excuse anything your WS has done, and he is still deciding to listen to someone that doesn't know you, over what you are telling him you need.
My advice would be to dig down and figure out if you can R with him doing that. Are you willing to leave if he doesn't give you a timeline? How about separate until he gives it to you? I felt very strongly there was no chance without a timeline. I need to be sure he was facing what he did, and I couldn't take the discoveries that were still happening 4 months after Dday. I also needed to get my agency back. He needs to be 100% in for R, if he isn't, don't give it to him. You can watch for progress, but withhold R until he earns it, or you give up waiting around for him to get there.