This will depend on what results you are seeking from yoga. The thing to remember about yoga is that it's not just about the results but also about getting there. If you want to see changes in your body, you may see results in as little as a few months or as long as a year. Again, it depends on what you are trying to change.
If it's inner peace, spiritual growth, and a stronger connection to self that is being sought out, that may take many months, even years. Some may feel an immediate sense of relaxation, but that's only the start of what you could attain with regular practice.