I take Ryanair flights from Stansted reasonably often (and Iuse the online check-in). From the terminal entrance, it takes me about 5 minutes to get to the end of the queue for security checks. (However, I know where I'm going. I'd advise a stranger to allow nearer to ten minutes).
The length of time for the security checks varies depending on how busy the airport is. Typically, they take 10 to 15 minutes. (I've been through in around 5 minutes but I've sometimes waited 20 minutes or longer). Once you're through security, the airport authorities advise allowing at least 15 minutes to get to your gate. So, I'd recommend allowing an absolute minimum of 45 minutes from the terminal entrance to the gate.
The gates are sometimes held until about 10 minutes before take-off but I wouldn't rely on it. (I once boarded a flight to Genoa 11 minutes before the take-off time. The door was slammed shut as soon as I was through and the captain commenced 'push back' before I'd found a seat).
The last time I travelled with Ryanair from Stansted, I arrived at the terminal entrance 1 hour and 10 minutes before take-off; I wasn't at all worried because I was confident that I had enough time. This proved to be correct. I was through security in just over 5 minutes and arrived at the gate nearly 40 minutes before take-off. However, if I'd have had under an hour left on arrival, I'd have been worried that the security checks might have taken much longer. (I seem to travel at 'off-peak' times. I've heard tales of security taking over an hour but I've never experienced anything anywhere near to that sort of time myself).
Chris