Need more info to answer this. I know you have been running off and on for a long time but more specifically, how about the last 6 or 12 months? What are your PBs (and what were you doing training-wise) and current estimated times. If you are re-starting as couch to 5K, you can drop minutes in a matter of weeks. If you are at 60 miles a week and have been for a year, it might take another year at 65 and some training tweaks to your workouts to improve 10 or 15 seconds. There are a lot of factors.
But in general, say you are at 10 miles a week now off of 3 days a week, and want to improve some. Double that over a few months and add in a tempo run and a set of strides every week your time in a 5K can drop a minute or so over the next half year or year as long as you are being consistent.
