I've had mixed experiences with fantasy scenarios. I tend to find that once my mind has started working down a particular avenue it just keeps on going, adding more detail as I daydream. I generally have a particular girl in mind while thinking these things up and I've had some amazing times and some... not so amazing.
The trick is to find a girl willing to put the effort in. There have been many occasions where I've outlined a detailed scenario and the girl has enthusiastically agreed, only to meet me at the door not even in the outfit I'd clearly requested. This is deflating and although at times this has still led into a decent session, it is always disappointing not to get that Christmas present you have dropped such heavy hints for!
On the other hand, when it works it can be amazing. I had an incredible fantasy session with an American girl, Lily Hart, who was absolutely word perfect and really got into the scene. The lovely Alix Dee (not working now) was also delivering a sensational performance before, half way through, she completely forgot what to do next. We just carried on and had some pretty great sex and then had a good laugh. I think it's important not to take it too seriously and be flexible.
I'm not sure the time factor is important. I've had disappointing one-hour scenarios and a couple of cracking 15-minute quickies. The key is the girl, as you might expect.