Time employed for writing
As a teacher who has taught many non-native speakers subjects in English, I am quite confident when I say that the time will vary by the person. Some people are quite strong and write faster (and better) than some native speakers but others write very, very slowly.

To have anything meaningful, there must be some quantifiable tie back to English level (what does 'good knowledge of English' mean to you? IELTS 8?)

It seems you are interesting to know if you are performing well enough compared to some standard, but we would first need the standard (and it does look like you're trying to find the standard) and that standard must take into account more than just the information you've provided (how much experience do you have writing in general? how good is your English? what kind of writing are you doing?).