Can my employer own my personal work made outside of work hours

I have just been offered an interview for my first Graphic Design job outside of commissions and college, but I had a quick question about ownership rights for artwork. In my spare time, I create artwork and designs not only for personal use, but for one-off commissions as well. Does my employer own the rights to these when they are made while employed? Here are some scenarios for example

When to tell long-term client I’m retiring within the next year

For fiteen years I’ve been designing a quarterly forty-page publication (print & online) and numerous other smaller projects for a small city. Now I’m planning to retire in the next six to twelve months. I haven’t yet told my client this. How far in advance should I do so? And when/how do I approach the subject of selling them the native InDesign files?