At what point should I no longer include my former professor as a coauthor?
I'll copy and paste my answer from another question earlier today:

Everyone who makes a sufficient contribution to your paper for authorship should be an author on your paper (and no one else).
It doesn't matter what institution they are at nor does anything else: everyone that contributes at a level appropriate for authorship is an author.

Probably the best person to judge this is your former professor. Have a conversation with them about where the project is at and how you see their contributions and ask whether they think it is appropriate if they are an author or if you should be a sole author.