I don't think this particular concession is unreasonable, but I wonder if similar concessions would be made for those of other faiths, or those of no particular faith who make other reasonable requests.
If I asked to be able to visit a quiet room at work three times a day to preserve my sanity, I don't expect I'd be taken seriously. If I asked for the same concession for religious reasons, particularly if I was a Muslim, I'd be much more optimistic.
Requests seem to be taken much more seriously if they are required by religion. Tell people you don't want to 'celebrate' christmas and they still go on about it, tell them you're a Jehovah's Witness and don't celebrate christmas, and all of a sudden it's acceptable to them.
Comparing praying to drinking isn't really fair, as prayer doesn't necessarily impair one's ability to work, but I think it gets a bit more interesting when you compare having the occasional drink or two to observing Ramadan.