Slingmedia recommend that you have at least a 1Mb upload speed where your Slingbox is connected. Which I guess means they recommend at least a 1Mb download speed from where you are watching the Slingbox. However it should still work, just the bit rate of the stream will be reduced.
IIRC (I haven't used my Slingbox for a while) there are options in the Slingplayer software where you can adjust resolutions etc to optimise your viewing for your connection speed. You can have a higher resolution which will look quite blocky or a smaller resolution which can look quite sharp, it depends on your preference.
Using your 512Kb download speed as a guide and assuming you can at least match or better that in terms of upload where the Slingbox is connected, the amount of data downloaded in an hour would be:
60 (seconds) x 60 (minutes) = 3600 (seconds in an hour) x 0.5Mb = 1800Mb (in an hour).
1800 / 8 (bits in a byte) = 225MB.