How young are your kittens? Age could have something to do with them not
using the boxes. Usually having an older cat around to show the kittens
proper cat behavior is more helpful than having another kitten. But, don't
fret. If you don't want another cat, here's what I suggest:

Use clumping litter in your boxes, not clay. The kittens like the fine
litter better, it's more like the consistency of dirt they'd use when
outside.

Are the kittens going in particular areas? If so, put the litter boxes in
those areas that they are frequenting, even if it's in the middle of the
floor. Gradually, once the kittens use the boxes, you can move the boxes off
into the areas you want them to be, one foot at a time. Usually moving them
about a foot every few days is gradual enough.

For now, put the litter boxes wherever the kittens are going on the floor
the most and leave them there until they are using the boxes every single
time they go. Whenever a kitten goes on the floor, if it's #2 then use a
tissue, pick it up and put it in the litter box. Cats prefer to go in a
litter box and like to cover their waste so this shouldn't take too long for
them to learn. Once their waste shows up in the litter, they'll investigate
it and eventually start going in the box. Make sure you clean up their urine
and whatever waste that ends up on the floor. Your kittens are not going to
be neat about this. Just let them investigate the box and let them dig. You
cannot teach them to be neat, just teach them where it is appropriate to go.

Do not yell or admonish your kittens for going on the floor. Do not separate
them, it increases their anxiety (they are too young for time outs), and
will cause the problem to worsen. Be kind, gentle and PATIENT with your
kittens. They are at a disadvantage because they don't have an older cat to
teach them. The best thing you can do is to PRAISE them whenever you WITNESS
them going in the litter box. Cats don't understand praise for something
they've done in the past. If you consistently praise them with kind,
soothing words (preferably using the same phrases each time like, "good
kitty, Max") whenever you witness them doing the behavior you want, then
they will learn what you want.

There is no right or wrong period of time for them to learn to use the
boxes. Surf the net to find out more about how to train cats and for more
info on your specific problem. Good luck!

PS- Also suggest you use small litter boxes, several of them. Either keep
the kittens in one room while they litter train or keep more than one box in
each room they are allowed to run around in. Make sure all boxes are either
where the kittens have gone on the floor before, or out in the middle of the
room where they can easily see and get to them. Eventually you will be able
to gradually move the boxes to a more permanent spot. If the kittens revert
to going on the floor at any point during this process, be patient and keep
working with them. They will get it, eventually :)

What worked for my three cats so far was using an shrill higher pitch word
that sounds harsh when they get it wrong, and a soft kind sounding word for
when they get it right. When you see he kitten mess on the floor, use the
shrill word, place the poop in the box and put kitty in the box.  When they
get it right, use the praise word and give them a cuddle and a treat.

Using this approach we have taught the cats that jumping on the lounge,
scratching the carpet, using the kitty box and jumping on people is not
allowed and they don't even try anymore.

This approach has worked well to me the the point where many other words are
understood by the cats to have different meanings. I use the word "tucker"
for getting the cats in because it means feeding time for them for example.