So long as your feed doesn't expire, the time in-between publishing is inconsequential.
Publishing every minute, every hour, every 4 hours, 8 hours, or 24 hours makes no difference, if the the actual price has not changed.
Your script should be checking the price often but only publishing if its changed some fraction of a percent from your last published price.
If the feed expires after 24 hours, at a minimum you should obviously publish at least every 24 hours. More often only when the price has changed.