It'll depend much more on your own packing style than on the length of the trip. Some people carry 40 lbs of stuff on an overnight outing while others manage with 15 lbs to go 3000 miles. So get together the things you plan to take along and see how well they fit in your current panniers.
Bike handling is usually better if you have some of the weight up front, so if things are tight with the panniers you have it's probably best to supplement them with a front pair rather than getting bigger ones.