Typically people recommend feeding 10-15% of the snake's body weight ~5 days for young snakes, and ~7 for adults, except around when the snakes shed (though mine has eaten in shed before, so I always offer). Sometimes adults go off feed for a couple of weeks, and that's normal. It depends on the individual.
I would read the care sheets here, rather than rely on the pet store for advice. This site is great because you're getting info from the biggest names in the industry. With pet store employees their experience is always a gamble.
There's also a lot of good advice here about how to switch a snake from live to F/T. Some keepers choose to feed live anyway, and some snakes can be especially picky with what and how they eat, but feeding F/T is doable if that's what you'd prefer.