I think it is a great question. Your 100 time is a good an easy measure of swimming progress. During practice a few months ago I swam a 1:28 for 100 meters and 3 weeks ago I swam a set of 10 averaging 1:34 on a 2:30 interval. Yes, plenty of rest on this set.
Last week during my warm down I decided to push it and swam 200 yards in 2:53.
I've been at TI for 9 months and have seen steady improvement despite being in my mid fifties and I think those times are not even close to my potential.
When I feel improvement I like to go to the pool and test it to see where I am. Time is the best measure.