Formal:
* Elle dort beaucoup. (This is the most literal and common translation.)
* Elle est très endormie. (This emphasizes the state of being sleepy.)
Informal:
* Elle dort comme un loir. (This is a common idiom that literally means "she sleeps like a dormouse" and implies she sleeps deeply.)
* Elle a le sommeil lourd. (This means "she has heavy sleep" and suggests she sleeps deeply and for long periods.)
* Elle est une vraie dormeuse. (This means "she is a real sleeper" and highlights her tendency to sleep a lot.)
To emphasize that she sleeps too much:
* Elle dort trop. (This literally means "she sleeps too much.")
* Elle passe beaucoup de temps à dormir. (This means "she spends a lot of time sleeping.")
The best translation depends on the context and the specific meaning you want to convey.