In order to know just how common infidelity is, you have to start by agreeing on what we mean when we talk about infidelity, adultery, or cheating. This has been one of the biggest obstacles for researchers trying to answer the question of how often people commit infidelity when they are in a committed relationship.
In a thorough review conducted in 2010 of 31 research studies on infidelity (which were conducted between 1982 and 2009), the responses varied so widely as to make generalizations impossible. The range across all studies was from a low of 1.2% of respondents having engaged in infidelity to a high of 85.5% of participants. An earlier review of studies on what was then referred to as "extramarital sex," which included research from 1948 to 1981, reported a range from a low of 18% to a high of 69%. The variation in frequency depended on many factors, both known and unknown. The authors of the more recent review suggest that the variability is largely due to the research methods, such as who was asked, what time frame you were asking them about, and how narrow or broad your definition of infidelity was.
While it's much more fun to say things like half of all people cheat, or infidelity is on the rise, a careful consideration of the research leads to only one sure conclusion: Infidelity, however you define it, is extremely common among some people and relationships, and less common in others. The research also suggests that there may be social or cultural norms that impact how common infidelity is. This makes sense, since sexual behavior and sexual definitions are linked to social and cultural messages and norms, and it would follow that the way we describe sex and have it outside of committed relationships would be linked to those norms as well.
Luo, S., Cartun, M.A., & Snider, A.G. "Assessing Extradyadic Behavior: A Review, A New Measure, and Two New Models" Personality and Individual Differences Vol. 49 (2010): 155-163.
Thompson, A.P. "Extramarital Sex: A Review of the Research Literature" Journal of Sex Research Vol. 19, No. 1 (1983): 1-22.