What's the bake time on the original cake? It will probably be longer than the original by about 1/4 so if you start your testing after the original bake time and go in 5-10 minute increments you should probably hit it with some accuracy. Now, keep a record of how long it takes in total and then take off about 3-5 minutes for each time you opened the oven door to test. So the next time around you leave it for the new time, test and adjust upwards if you need to.
A lot will depend on the denseness/moisture content of the cake and how long it takes the thickest part of the cake to cook. That said a bundt pan won't take as long as a loaf pan and is likely to be relatively close to your 9/13 at the very center where it takes the longest to cook. Your results may even be better because there won't be any corners overcooking and getting dry. Good luck and let us know how it goes. If it's very good, post the recipe please.