the Flurry SDK docs say the following for logEvent():
Use onEvent to track user Events that happen during a session. You can track how many times each Event occurs, what order Events happen in, as well as what the most common parameters are for each Event. This can be useful for measuring how often users take various actions, or what sequences of actions they usually perform. Each project supports a maximum of 300 events, and each Event ID, parameter key, and parameter value must be no more than 255 characters in length. Each Event can have no more than 10 parameters. The parameter argument is optional, and may be null.
So the limit seems to be 255 characters – although I think you should keep the event names really shorter as you will otherwise loose overview in the web analytics tracking 😉