There’s really no short and sweet answer to this question because it depends on several factors:
Previous knowledge
How much do you know about Kannada? Having initial knowledge of some common terms and the character sets may be enough head start for you to read, write, and speak like a native.
Time spent learning
Nothing beats hard work and patience in learning a new language. As long as you commit to spending a significant amount of time getting comfortable with Kannada you can expect better results in the quickest time possible.
Method of learning
How you plan to learn Kannada can make a huge difference in the speed of acquiring the language.
Your purpose for learning
One of the fastest ways to learn Kannada is to determine your intention for reading or speaking the language. If you focus on Kannada words and terms normally used in a specific context or situation – say, conversational terms for travel or business – you will sound like a Kannada local quicker and more convincingly.
If you’re really looking for number estimates, here are some projections on how long it may take an average person to learn Kannada
Street conversations or small talk: 1 year
Higher-level conversations on business, politics, and the government: 4 years
Reading emails, social media posts, and short articles: 2 years
Reading full newspaper articles and local books: 4 years
Understanding movies and TV shows in Kannada: 2 years
Understanding Kannada music or standup comedy: 4 years