Its about a 4 hour drive, from Monteverde to Jaco to the ferry, its about 90 minutes from the ferry to Montezuma. The roads are ok to Jaco but on the other side of the water the roads can be rough, I would try to time it so I arrived with some daylight left, some of those switchbacks acan sneak up on ya
Correction to the above, you are right in thinking you must get to Puntarenas for the vehicle ferry. I'd give it 2.5 to 3 hours to make the drive. Hopefully someone will chip in with latest ferry schedule, but get there early to get in line.
FYI, Not certain if you have a car. But there are van shuttle services (like Montezuma Expeditions and others) that will do such a trip for about $55 a person. They usually get you at your hotel and one end and drop you off at the other, they cover the ferry fee. Usually you transport your luggage onto the ferry and board another shuttle on the other side, the advantage of this is you don't don't care how long the car lines are since passengers just walk on.
The roads are not super bad, but a 4x4 is recommended if rented. I rarely rent, I usually shuttle around.
We drove it ourselves. I would leave plenty of time to get in line for the ferry, buy a ticket, etc. look out for scammers who offer to buy your ferry ticket for you and then demand a ridiculous 'service fee.'
Road from Monteverde to the Pacific is horrific and frightening. 4X4 is almost a necessity--be sure your rental agency allows it.
This link covers it pretty well, though you'll want to confirm the timetable.
Again, I'd recommend the shuttle service, unless you need a car for something you didn't mention; unless you already have a car reserved. These shuttle services can even get you to Monteverde too :)
Racandee, very surprised at your comments 'horrific and frightening'. Certainly the first stretch back to the paved road can be challenging for some, but from there to the coast its a very easy drive.
Personally, I would take the bridge across to the peninsula if you are in Monteverde (the road down the mountain to the highway is very close to the bridge across the gulf), drive to Paquera and Montezuma. When you are on your return, if you are heading for the San Jose airport, take the ferry. That way you don't see the same scenery twice and you get to see some countryside other than the main highway.
That's a good idea with the bridge, I didn't realize how far north Monteverde was (I've never combined it with a Montezuma trip). That's actually a pretty cool long bridge.
Simon P--the unpaved stretch is like driving on a river bed. With 1000 foot drop offs on the side. With no shoulder or guard rails. But with cows.
I would never drive it again.Edited: 7:53 am, January 27, 2013
The first section of the road on the sta Elena to sardinal route is by far the worst bit of road I've ever driven on. This is 4x4 only because you'll need the ground clearance. Like racandee said, it's like a river bed. It does eventually turn into paved road but it is narrow switchbacks, very steep all the way down. Beautiful views though!