Hi Mike,
It depends on the timeframe and business question(s) you're trying to answer. For example, if the question is how many leads enter a specific stage (e.g. MQL) in a month you'd need to have at least one month of data. If the question is how many days does it take on average for a lead to go from Known to Opportunity you'd need to have at least full sales cycle period.
If you have really long sales cycles the top of the funnel (TOFU) metrics will be more relevant.
There is a seasonality factor to that you'd also want to take into account.
TJ