When should you? Not until you're sure its the only course of action left. Keep in mind, if you do this wrong you're very liable to do more harm than good in the end. Are you prepared to cause that much stress on your snake? My calico pastel took a month to eat for me, my cinnamon took almost 3 and she still only eats once a month or so now. Everyone has their problem eaters, the best thing for them is patience and to leave them be in peace. Odds are your snake will not starve itself to death and if you keep up offering food eventually she'll take it.

By the by 600-700 grams is terribly small for a female over 2 years old. I have 3 born in 2013 and the smallest of them is close to 800g, the largest being over 1100.