Progression of times in SE

What type of progression should I be looking for during the span of my SE during each training period. For example the 300’s in the SPP and 200’s or 150’s in the precomp, what type of progression in terms of time should I be looking for. What is a normal improvement from the first session to the last session, percentagewise (x % of PB to y % of PB) or timewise (x seconds improvement). Should there even be a vast improvement or should one start SE only when you’re ready to run close to your best (and thus the improvement will be small)?