The ROI will vary greatly for each keyword, ad, landing page, and bidding strategy. You shouldn’t be concerned with maximizing ROI, that’s the wrong metric for optimizing PPC campaigns. Maximizing profit per month is a much better metric.
A minimum ROI is sometimes established to allow for working capital restrictions, especially with products or services that have a long lead time, or longer sales cycle. In those cases, there may be substantial operational expenses that increase the cost of financing your marketing efforts. However, in many cases, you should not be concerned with the ROI metric.
You should establish an average value per sale for each product/service you are marketing, then focus on maximizing total profit per day/week/month. You do this by a process of continuous testing and optimizing your ads, landing pages and bid levels for each individual keyword.
The actual strategies and techniques for optimizing each of the above processes could fill volumes of books. While many PPC marketers have been successful with relatively minimal knowledge of those processes, it is becoming more difficult each day due to the competitive nature of auction-based ad positions.