This is very anecdotal, so YMMV, but when I go from a long period of recovery (off season, injury, etc) and back into regular training, my resting heart rate typically drops about 10-15%.
Training-wise, the largest reductions in resting heart rate usually follow training periods that include max sprint sessions supported by lots of easy volume. I first read about sprints’ influence on stroke volume in Renato Canova’s writing. I then paid more attention and noticed it in my own training.
According to Canova, near-maximal sprints “shame” the body into increasing stroke volume to better its reaction to sudden, intense loads. For me the change usually happens over six to eight weeks.
However, it’s also worth pointing out that resting heart rate isn’t relevant to performance. It’s worth tracking as a recovery metric, but it won’t inform your training very much. I put RHR in the same bucket as VO2max; it’s a “cocktail party metric.”
It’d be much more beneficial to keep track of your speed at aerobic threshold (AeT) or at anaerobic threshold (AnT). Changes in those are very relevant to performance.
I hope that helps.