If you only upload web resolution images to flickr it will take ages to use the monthly quota. Plus it is annoying wading through hundreds of images that are very similar, which seems to happen a lot of flickr...
That will be why your 100Mb is going down quickly, I only upload to Flickr at 640px 72dpi, you can go bigger, but there isn't much point in going bigger than say 1000px at 72dpi.