I'm currently writing my website in MYSQL but I know I'm going to switch to Cassandra. Can I use both or should I just choose one over the other? How hard and how long did it take to switch from mysql to cassandra (less than 5 minutes or longer)?
Dependant on your code logic, you could, and possibly should use both - using mysql where it suits and then jumping into Cassandra for the data storage?
I've been considering a solution for one of my projects, balancing both mysql AND a nosql 'data store'.
Depending on how you structure your application, it may be easier to start development with the final setup, but equally if you structure your code sensibly it'll be easy to switch from one to the other later.
You do know that cassandra is NOT a relational data store so if you are doing relational stuff it really doesn't map, right?