I usually don’t go to complete failure, and I don’t think it’s the best idea to go to complete failure every single set; you want to be able to do a good number of reps the next set. If you want the fastest results, you have to either focus on strength training or building endurance; it will take longer to improve both. On this forum I typically suggest focusing on strength, keeping the rep ranges low (anything past 20 and you’re well into endurance territory).
Beyond that, you’re overthinking it man. Just train. Everyone reacts differently, the only way to know for sure how long it will take is to do it. For the strength goals you listed, for most serious trainees who start out with high relative strength (such as people who are relatively small and/or young), I’d say 2-6 months is a reasonable time frame, but it will take longer for some people.