I can't quite remember how long it took me to finish it either, but I want to say it was 3 weeks or so for the first time I completed it. I probably finished each section in one week (GC, OC, Bio). I did not do much of the QR section in Destroyer because I replaced it with Math Destroyer. I was able to complete the GC and OC sections twice and the second time through went much faster, like within a couple days! Some questions I even went under a third time to strengthen my weaknesses. However quickly you go through the Destroyer is dependent upon how much time you have and how determined you are 🙂
If you're comfortable with making flashcards, go for it. I realized flashcards were not my thing when it comes to the Destroyer. For GC and OC, I just did the problems over and over again until I understood. What helped me solidify my understanding of certain subjects was to categorize every single question under what topic it belongs to. I made a giant spreadsheet on Excel and that helped me a lot. For example, some topics would be "percent yield calculations" or "stability" or "gas behaviors" and I would gather the corresponding question numbers under those headings. It took forever to make, but well worth it because then I'd know which topics I had trouble on and had an index to follow. For knowing OC reactions, instead of flashcards I folded pieces of paper into two-thirds so that the flap on the outside either showed the reagents and underneath the flap I had to finish the reaction, or the flap on the outside showed the product and under the flap would reveal the reagent. As for bio, I don't really have anything useful to say. I just read, absorbed, and hammered them into my mind, literally lecturing to myself the topic verbally of what I just read from Cliff's. Talking to yourself helps, for me at least! I also wrote down anything I wanted to clarify and get down completely of things I didn't understand in a small notebook because writing/drawing helps me memorize too. Just simply going through those 500+ bio questions from Destroyer helps solidify topics as well since you'll see many of the same subjects repeated.