No, I don't think 24 hours is necessary. And I say that as someone who is currently in the "long wait" between interview and decision. My interview was Tuesday and they said they would give me a decision on Friday. I think that's acceptable, especially since - and this is crucial - they specifically told me Friday, so I haven't spent the last couple of days on tenterhooks.
I think being given a clear time frame is very important and I think maximum of a week for a decision, unless they can give you a good reason why not earlier.
I once went for a job I really wanted. I spent three full days really researching hard and preparing as best I could for the interview. They also wanted me to do two pre-interview tasks which I completed and which was basically hours of work that I did for them for free. The interview was two towns away so there was a bit of travelling and expense involved too. I bought a brand new outfit.
I thought the interview went well and I know they were impressed with the tasks. I asked how long and they said "week after next" which I thought was a bit crap, but what can you do?
Week after next came, I waited until the end of it, and nothing. I sent a polite follow up on Monday, nothing. I sent another follow up the following Monday... tumbleweed.
After another week I sent a professionally worded but very cutting email telling them exactly what I thought of the recruitment process and how rude I thought they were. 7 minutes later, I get an email back offering me "a chat on the phone" stating she was available all day and what time should she ring me, I gave her times and she... never even rang me. She then emailed back a day later to say, sorry, was busy, please call me whenever you like. At which point I gave up bothering.
That sort of thing is never acceptable it's RUDE AF and show such a lack of professionalism and organisation.
And the worst bit? This was a job in an MP's office!!
Aren't they supposed to care more than anyone about their image??