I was working with a dataset that had a 'period of stay' feature and had a value like Mar-May I want to convert it to numerical form

you can probably convert it to a date format and then do a difference between the two dates. You would, of course, need to convert the string "Mar-May" split by the dash into "Mar" "May" and then conver the data.

I'm not sure what programming language you're using, but I'm pretty sure the one you're using would have some date data type that you can use that understands time arithmetic to solve your problem.

Hope that helps. Good luck!