The phrase "plenty of rest" is correct and usable in written English. You can use it whenever you wish to emphasize that someone should take a lot of rest in order to feel better. For example: "I hope you are feeling better soon, and don't forget to get plenty of rest.".
Still, at least Sehwag has had plenty of rest.
We had an opportunity to get plenty of rest".
Then she asked Dr. Scheinin if he had gotten plenty of rest.
Get plenty of rest.
Make certain that your children get plenty of rest.
US flu.gov webpage recommend getting a plenty of rest as a treatment on flu without medication [ 1].
Have plenty of rest.
I love the desktop app, it’s always running on my Mac. Ludwig is the best English buddy, it answers my 100 queries per day and stays cool.
Cristina Valenza
Retail Lead Linguist @ Apple Inc.