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I've always wondered how long it took other sound designers to finish a single sound effect. Sure there are those 2 min wonders, but I know sometimes a sound effect can be so frustratingly obscure (I work in videogames) that it takes me more then a few hours to get it right. I think it usually takes me anywhere from 20-30 min on average for a sound effect.
Dont forget to factor in the recording session time if you had one.
It so depends. Depends on everything. I've recorded for hours to get one sound, and recorded for a few minutes and got dozens of sounds. Editing-wise, the same applies. It also depends on how much I know about what I need to accomplish, and then subsequent revisions based on client input. I think all of us have moments when the planets align to get a decent sound designed in <30 minutes, and moments where it seems to take hours...and sometimes it really does. (all this including recording, library diving, mixing, a bit of editing to picture/event, etc. etc.)
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I think this ties in very closely with a Randy Thom interview I saw recently on the Designing Sound website (I think it was called 'Meet the CAS Winners').
Basically he talked a bit about the need to not be afraid of making mistakes, especially early on in the project when there's time to recover from any dead end paths you may have taken. I hope I'm not distorting Randy Thom's comments here but in this way I think it is important not to set yourself too rigid a time limit to produce an effect - if it's an important one of course. I don't know exactly what your time constraints are, but think about what the big fx moments are in the project and then give yourself a bit of room to breathe with those: experiment, take a risk, follow that hunch. If necessary, you can recover some lost time by being a bit more methodical with less important fx. But if you're not flexible with how much time you allow yourself, you'll find yourself saying 'that'll do' a lot which, as well as not producing great work, becomes very demoralising for a creative sound designer.