It is very individual. I started to get some contact with my triceps after a couple of weeks, but I have a incomplete injury. I fist managed to get from floor to chair about ten months after my injury. Hope you'll get better and that you'll find a good way to manage this.
Good work. I tried it to get on my wheelchair before watching your video and I couldn´t do it, I think I should get stronger triceps as I´m a c6-c7 complete.
That's the hardest part... I get much help by using the handles on the backrest as shown in the video. Hope you'll manage to do it soon! We all have to find our own tecniques as every spinal cord injury is unequal. I'm glad to hear that my video can be helpful :)