this seems to be a fairly simple question but i can't quite get what im looking for. hopefully someone can help me spot what is probably an obvious error.

I am trying to find the RL time constant of the following circuit. I turn off the switch at t = 0.5s and measure the time it takes for the inductor voltage to go from its high to "zero". I believe we can approximate the time constant of the circuit by taking that time (5.03e-3 in this case) and then dividing by 5 to get the value of a single time constant. This value is about 1000μs.

However, my calculation for this circuit uses time constant = L/R. using the values for my circuit, that gives me 10m/101, which is about 99μs.

thanks in advance for all replies.