My family always buys park hoppers. For us the being able to go from one park to another is a great way to break up the day. Plus, if one park seems really busy, you can always check out a different one.
To answer your question OP, allow about an hour to get to one park from another. If you have an ADR, Disney suggests you allow 90 minutes, just in case you r un into any bad luck with traffic, or the buses/monorail/boats not showing up in a timely manner.