This talks about a study showing use during a 60 minute workout every 3 days for 6 weeks made a big difference. I’m not clear if the horse was worked more than 3 days/week, the others without the bell boot.
https://thehorse.com/178062/crooked-horse-under-saddle-a-single-bell-boot-can-help/
As for how long - it likely depends on how long it’s used each ride, and the degree of asymmetry to start. If it’s something you can feel, then you’ll feel things getting better, but even then, it’s a good idea to take regular video to more objectively evaluate things. I’d do weekly or at least every 2 weeks.