Your child should continue to sleep with their plushies as long as he likes it! Children have their own schedule for growing up and its much slower than some adults have the patience for.
By 5-year-old most kids will no longer need a comfort object like a plushie or blanket everywhere they go. But they may still sleep with that object for much longer. Many kids will stop sleeping with their plushies by 10-year-old. No matter how long it takes the behavior is normal and is simply a token that your child uses to reassure himself everything is ok with the world.
Many studies suggest that any kind of comfort object should be encouraged for kids until they feel like they don’t want it anymore. For example, an 8-year-old will not have developed full emotional maturity and stability until late high school or college.
The time she starts itching to move out will be a beginning indicator that she is close finishing the development of her emotional maturity. Until that point, a comfort object may help illuminate or dull emotional outbursts. Keeping a child more stable by giving her an outlet and comfort when she is struggling to use the proper self coping tools to soothe herself.
Taking the object away that is providing her this comfort creates a level of resentment and distrust. Also can psychologically play a roll in training her to be emotionally detached down the road.