One of the requirements of filing a motion to set aside a default is that you have to attach a proposed Answer to the motion. As long as the original lawsuit was filed within the statutory period, you will have to defend it. Even if the original lawsuit was not filed timely, you will still have to defend it, but you will have an affirmative defense of the statute of limitations.
As for how long you have to file the motion, I have filed motions like this for clients even several years after entry of judgment. You just have to convince the judge that you had no notice of the lawsuit, could not have gotten notice any earlier, and the you filed the motion as soon as possible after you found out about the judgment.
If the service was fraudulent, then you can set it aside at any time after entry of judgment. But you still must move diligently. I am concerned that you have waited several months since hearing of the judgment before doing anything. Generally you cannot wait more than about two months without good reason.
As noted by my esteemed colleagues, the window is open on setting it aside even 10 years later IF you act swiftly after finding out. There are attorneys who specialize in this area of law, mainly those who handle debt defense, since slimy debt collectors cheat the system regularly. I recommend giving Stephanie Tatar in Burbank a call. Tell her I said hello.