I admit, I was on the fence about this one. I originally contacted our hotel asking to book Tabacon hot springs and was offered this as a less expensive, less crowded experience since the number of guests are limited by reservations. At our hotels suggestion, I waited until we arrived to book and we almost didn't get to go, because all the nights were already booked up except for our last night in Arenal area! So you may want to book this in advance. We had thought since it was off season it would be no problem. The cost was $34 per person for entry to springs (receive access to locker and changing rooms/facilities) and then we chose the additional $17 per person option of dinner.
We were traveling with another couple and our 11 yr old son, and we enjoyed our experience very much. The bartenders could stand to be a bit more friendly (very grumpy and stand offish) but I must say the cocktail drinks were absolutely delicious and made the right way! (this was a nice change) At dinner you got your choice of meat and we all chose something different. I got the fish and was extremely happy with my choice (tilapia) with rice and beans, and the BEST salad I had on the entire trip. It was amazing--full of avocado and hearts of palm. The meat came out in a bowl for each individual, but the sides were served family style so you could ask for more.
Three to four hours (with dinner and drinks) is plenty of time. Very relaxing to soothe a body after a strenuous hike. While on our visit I asked other travelers and heard positive feedback on Tabacon also (but more expensive) so I would probably try that one next time just to have tried both. But I don't think you'll be disappointed either way!