Show me the magic!

[Shostak]

It's a neat idea for a spell. "Fasted" doesn't seem like a clear description. Perhaps "hung" or "conditional" would be better?

Also, I'd recommend a small set of easy durations (minute, quarter hour, half hour, hour, half day, day, week, month, season, half year, year) with associated costs so as to obviate calculations at the table.