Tocanadawego said:
Exactly what I've been saying too! If you know it once, you know it for good.
To be fair, I think that depends on the person and his/her situation.
If you're a native speaker or live in an English-speaking country, you're certainly right, but if you learned English as an adult and don't have a reason to practice it every day, all four skills, then your knowledge will deteriorate pretty fast.
I passed a Japanese language test about three years ago, and didn't have the opportunity to practice it since then - I don't think I'd be able to speak even about the weather today.