How much video will a 8gb sd card hold?

I've got a Nikon 3300 DSLR, how many hours of 720p or 1080p video would it hold?

Read the manual:
http://download.nikonimglib.com/archive1/9gsPT00RHvMa008gIk986r0uve63/D3300_NT(En)01.pdf
Recording movies starts on page 35. File size limitation is on page 38 along with the maximum amounts that will fit in the file size limitation. More like minutes rather than hours. Of course, that assumes the camera does not shut down from overheating, first (see pages 33, 39 and 40).

If you insist on doing video, at least get memory cards with more memory.

So sad you got a device designed for still images to do video. Your dSLR can get good video, but since you have to ask the question, chances are pretty high that it won't.

The manual tells you in a chart near the back, based on the settings you've chosen.