What can you do if you accidentally used interior paint on an exterior surface?

It depends on the quality of the interior paint. If it's 100% acrylic interior paint, It should be fine outside if it's not in an area that is prone to mildew. If it is, you'll need to clean it with chlorine more often.

If it is not a high end interior paint, it won't last long outside, especially on wood. If you're not in a cold weather area, you might get three to five years out of it. If you're in an area prone to freezing, it won't be that long. You need to repaint it while it is till a sound surface, otherwise you're making more work for yourself.

Your other option is to just paint over it with an exterior paint if you want to do something about it now. Depending on the quality of the paint, this may be your best option since there will be a minimum of prep because the surface is already clean and sound.