In the Output settings in OBS you can set the streaming video bitrate. A high quality 1080p 30fps stream is usually up around 9mbps, which would take 8 minutes to use up the 500mb (not counting audio).
In theory, a high quality 720p stream would then require around 2mbps that would spend 500mb after half and hour.
In your case you would probably want to stream in SD, or 480p at 30fps at around 500kbps with audio at 98kbps which would spend your 500mb after about 1.5 hours.
If you value audio quality in your stream you should cut a bit on the video bitrate and set audio bitrate to 160kbps or more