Hi, Santa Clara is about 3 hours drive from Varadero. From Havana it is about a 4.5 hour drive. It would be much easier to go to Santa Clara directly from Varadero. You can either take a private taxi or try to use the viazul coach bus.
In my opinion, one day in Santa Clara is enough. While it is a pretty city, it is small and outside of the tiny tourist area there is nothing much. But, considering the long drive from Varadero, you might want to spend a night in order to make the trip less exhausting.
Just tell me if I can help with anything else.
Take care.