Five years is unusually long, but your case is an outlier too. Usually it will take about 3 months to rise serum levels by ~30ng/mL on 5K IU daily, though there's plenty of individual variation of course.

It's a low enough dose that if I had to choose between taking it blindly or not taking it at all if I was so depressed I couldn't get out of bed to get my levels tested, then I would do it blindly for a month or two without second thoughts.