If you are going to use thumbnails, I would 
suggest maybe 2-4 seconds loading time per image. 
My largest gallery, my Outlaw Star section, has 
22 images that are each 200x150p in size. After 
purging my cache and reloading, it took about 40 
seconds to load the entire page on the first 
visit, including banners. Upon revisiting, it 
took 5 seconds. Each of my images is around 6-10k 
each. I would say anywhere around there is about 
as good as you are going to get. Hope that helps.