We really loved this hike. We left from Hacienda Merida, and the trail started just across the road. Many other groups left by 7am, but we waited until 8 and it worked out well. About 3.5 or 4 hours to hike to the rim. Guides are required (although we saw one group go up without one) and ours was great, very fun and informative, speaking to us only in Spanish (other guides are bilingual). We saw a large group of monkeys on the lower part of the trail and our guide also pointed out some interesting plants and flowers.
The hike was very hot for the first hour or two before we got into the cloud forest. But, wow... the cloud forest was incredible. So dense, wet, and green. There were also several nice views of the lake and Volcan Concepcion. We relaxed for almost an hour at the rim, and after waiting a while the cloud cleared and we had a nice view of the crater lake below. It is possible to hike down from the rim to the lake, but our guide said you'll need about 2 hours round-trip.
We were back at the bottom in time for a lakeside lunch and beer before sunset. Highly recommended hike.