This depends on how much you’re willing to invest but it is also highly dependent on the competitiveness of your market and the length of your sales cycle. It also depends on how you define results. If you have an ecommerce site with low-ticket products, and taking an increase in revenue as an obvious campaign goal, then usually we can expect to start seeing significant rankings increases and associated sales growth, in the last half of year one or start of year two.
For companies with high-ticket products and much longer and complex sales cycles, this can be a far more complex question.
When we have more information on your products, target keywords, average sales value and conversion rate, we can give you a pretty accurate prediction of how long it will take for you to start seeing serious money coming out the other end of the equation.