Definition
How long does it take for you to "payback" the money you've spent on acquiring a new user. A faster payback period is better because it means that you can reinvest that money into growing faster. Typically, a faster payback period means faster growth. At the same time, depending on your business you can use a longer payback period to give you the freedom to acquire users at a higher price. If you're only bidding based on a 1 month payback period, the amount you're able to spend per user will change greatly versus a 1 year payback period. This will be influenced by your free cash flow, desire to grow profitability, and maturity of your business.