For most, the effects of CBD, when inhaled, are noticed instantly. If you are not noticing any difference, I would recommend raising your dose of CBD and see if this changes things for you. I also recommend trying topical creams for your back and knee. Many find these more effective than inhaling the cannabis. Combine both for the best results.
The time it takes for your CBD to go into effect is going to differ from person-to-person. Generally you’re going to be looking at 20 minutes – 1 hour. How much you weigh, how much you’ve eaten that day, how much you’ve exercised that day, and a slew of other factors will effect the time it takes for the CBD to activate. You’ll just have to pay attention to how long it takes for you to feel the effects.