I was thinking of this exact image when I clicked the thread. Every learner is different, and it would depend on how much time/effort one puts in, as well as their natural skill. There's also the added factor of whether the language they're learning is close to their native language (or even another language they've learned before). Mandarin might be hard for an English speaker, but easier for a Japanese speaker. Likewise, English might be difficult for the Asian language speakers but easier for European language speakers.