Ultimately I believe you should listen to your members, if they complain it's too high then lower it.
I believe that XenForo by default sets the flood control at 180 seconds. I lowered this to 60 seconds after one of my members requested that I lower it, and I haven't had any complaints since..
I haven't failed. I've just found 10,000 ways that won't work.
I think 30 seconds is sufficient enough. It's not too short and will give moderators and admins enough time to deal with a spammer accordingly keeping the spam and/or 'damage' to a minimum. I feel like forums that have it for a minute or higher, it tends to break the flow of conversation and discussion. 30 Seconds to me, is plenty long enough.
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