The "Internet of Things" (IoT) has a large number of definitions depending on who you are talking to. That being said industry analysts and companies are doing a better job at breaking down the IoT market segments and tracking their growth over time.
Several years ago the market for connected products and services was promising eye popping growth numbers of up to 100 billion units. Today, a majority of forecasts show a more tampered 20 billion or 30 billion units (while a few others say we are saying we are still severely underestimating size of impact).
To sum it up there is a by consensus forecast, that IoT will have a growth of 30% by 2020 yet if you want further information, check this out for more information about Trends, Forecasts by Industry, Country and Data: