Hi Paul
It depends, Ok we go with simple maths & now Imagine
1 Module you want to automate regression testing
10 Total test cases are written
7 Test case you able to automate
3 Test case manual testing due to application complexity or tool not support.
Now
To run 10 test case you need manual 4 hours
Now you add 7 automate regression testing
Now total time reduced to 45 min for all 10 test cases ( including 3 manual test case )
Now
Releases depend on your development methodology/framework.
If you are using CI then you can calculate or plan how frequently we are running these test and how much time we can save.
Golden suggestion:: Get insights
Make a excel sheet / google sheet make some data analysis with your on going testing project with ( Total testcase, No of manual, No of automated, Release frequency, regression frequency, time spent ) you get your answer yourself.