How long (on the average) does it take for Aikido to be useful in against an all-out determined attacker?

How many years would it take for someone that studies aikido to defend themselves effectively? By effectively I mean that he can evade an attack or grab a wrist that grabbed him, throw the attacker to the ground, and then pin him. How long would it take to develop this level of aikido skill. I'm not looking for it depends on the person and how well he learns type responses. I am looking for if you are an aikidoka how long did it take you to have these skills and feel like you can use them effectively against a real, determined attacker? If your not an aikidoka, how long do you think it would take the average aikidoka to do this?