I think you will probably be amazed at how quickly they will remember the litter box! It is a cat's instinct to bury their waste, so litter boxes provide the ideal way to do this in a house!
There are a few ways you can make sure they do go though. First, as someone else said, there is the soil trick (if you use your own garden soil, just make sure to sterilise it in the oven first!). There are also products like Cat Attract which I know lots of people have had success with (an additive you put in the box). Our kittens were feral kittens, so I was really worried about them using the box - but all I did was put the box in it's spot, sat next to it and scratched the litter when they were looking. It got their attention and they came over to investigate. Within minutes, they were all using it like champs.