There's a big variety in the amount of help you are likely to get from a battery. I'm fat and slow so will need more help. Bosch have a "range assistant" which will help you calculate how far you are likely to be able to cycle.
https://www.bosch-ebike.com/en/service/range-assistantThere are plenty of variables in there but it's probably the best stab that currently exists for predicting the range.
If your normal speed is not far short of 15mph, then a battery will last you a long time - and is probably unnecessary anyway.
I didn't keep hold of the full details of my 38 mile ride of 17th March, but I do recall trying really hard to keep my average speed for the ride over 9mph. Compare that to yesterday's ride when the return trip was 45k and I averaged 19.5kph.