Many times, when asking speakers how much they understand of another language, a process known as determining mutual intelligibility (MI) of two languages, speakers will give you a rough impressionistic description of how much they understand of the other language, but they will not give you even an estimated % for how much they understood.
I have been studying the mutual intelligibility of different lects for years now. Most of my work has been in Slavic, Turkic, Germanic, Sinitic and Romance. Over that period, I have heard many offhand MI descriptions of two lects many times. I also heard a number of MI% estimates of the same two lects. In many cases a speaker would give me both an MI impressionistic description and an MI% estimates. After a while, I learned that some impressionistic MI descriptions lined up quite well with certain MI%’s.
Obviously the best way to determine MI between two lects is via a scientific MI study. However, these are not commonly done, and many researchers do not want to do them.
The typical situation for the linguist looking to estimate MI between lects is that no scientific study has been done on these two lects, nor is one likely to be done in the forseeable future. Linguists have to make use of the information we have at the moment, and linguists commonly give estimates for MI of pairs of lects that are not based on scientific studies. Raw MI estimates are used very commonly in Linguistics, even in academic Linguistics journals and books.
With that in mind, here are the results that I learned over years of MI study:
“I couldn’t understand it at all. I maybe recognized 3 words.” = 0-5% MI
“I understand almost nothing, no more than a few words.” = 5-10% MI
“I understood handful of words here and there.” = 10-20% MI
“I can’t even get the gist of it.” 20-30% MI
“I can get the gist of it or the general meaning but not much else.” = 30-40% MI
“It’s hard to understand.” = 40-60% MI
“I can understand it, but not completely!” = 60-80% MI
“I understood almost everything” = 80-90% MI
Some variation of “I understood it perfectly, no problem.” = 90-100% MI