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[boa]

I love these 'how long is a piece of string' type questions.
Opinions on this, like every other subject, vary considerably. A lot depends on the species you are talking about. Personally I feed most hatchlings and up to about a year of age every 5 days. Feed them a meal which make s noticeable bulge without really stretching the skin.
You don't need to wait until a snake defeacates as the frequency varies with different snakes and even individuals.
You can move up to pink rats when the thickest part of the snake is a little thinner than the thickest part of the rat.
These are my own opinions and what has always worked for me. Some will say feed less often and some even feed more often.