That's what would happen with my Caribbean-tinged Spanish: It wouldn't take very long for me to carry long a long spoken conversation. I can converse online in Spanish without much difficulty, but I don't claim full fluency simply because my spoken Spanish (and Portuguese) is so rusty that I'd struggle mightily for a while. But I will note it's more a matter of forgetting the words rather than forgetting the syntax.

I could have added Latin, but I rarely read in it these days, but the Vulgate is so simple that it's like reading a children's book compared to Cicero or Catullus.