When out field recording problematic ambiences I sometimes resort to closing my eyes & counting seconds (one thousand and one, one thousand and two, one thousand and three etc) and as soon as a problem occurs (I notice unwanted noise pollution etc) I stop counting and only resume once I have a clean ambience again... I do this to insure I have captured more than enough ie a useable amount of clean ambiences... But over the years I have noticed that sometimes if I set a process going and it estimates eg 23 minutes and I leave it to it, without looking at a clock (or watch or my phone) I will walk back in just as it finishes... So I have always wondered if this is a side effect of working with time based media ie you develop an instinct for duration
I find that when I am recording ambiences and walk away to prevent any potential disturbance from impeding the recording, I go and listen to music which I already am familiar with the duration and set my recording duration in respect to the time I have gone off to listen to music. I also have timed myself reading per page so that when I have read a certain number of passages in relation to the length of recording time, I return and find that I am within 30-45 seconds of my estimation.
I like your counting technique as well and if you don't mind, would like to use it for more problematic recordings where there is a higher chance of sounds intruding on my ambient recording.
It depends on my mood I am in. If it is a hot summer day with the sweat running all over and I'm recording in a stressful or loud enviroment (let's say a highway) I tend to think that more time has passed than it actually did. If I'm recording in a calm ambience where I feel very comfortable (beach, corn fields, etc.) time flies by and I tend to end up with a longer recording than I were up to.
I normally have my iPhone with me. I keep the equipment far enough away and monitor with headphones or just listen without them. I always check the starting time and start writing down notes on the specific session, basic stuff like airplane overhead, type of birds that whistle, but also weather conditions and how the scene feels. I also start recounting after a disturbance or conflicting sound in the scene. After i think i've head enough (and it never is enough :) i try to think of a good description and filename. This saves a lot of time back in the studio.
I just realised I didn't really answer the question. Here goes: It depends on the situation, when i'm not a specific spot but moving about i mostly don't count. I mostly observe and listen to the scene and when i get 'bored' i most of time stop recording. My experience is that if I'm not interested while listening at the spot, i will also not be using the recording very often. At specific locations I always use a silent timer to at least get a certain amount of material.
Most of the time I'm having a harder time pressing stop, simply because I fear on missing out a great new sound. I hate it when that happens..
I go more with specific ratios - usually 2:1, but sometimes as high as 5:1 if there's a lot of ambient crap going on.
In other words, if I feel like I need a solid 5 minutes of clean ambience I'll roll a minimum of 10 minutes (if things feel clean) and as many as 25 or 30 minutes if I'm dealing with mostly noise and only intermittent clean sounds.
Simple math, and frees my mind to think on other things.
I've also been known to measure recordings in candy crush / angry bird game iterations. :)
This all makes me very curious, and my approach is a bit different. I often find that when given a reference time from a clock, if I am asked within an hour after the reference is given I can give the time within a minute or two. This usually happens when I am doing something which calls strongly for my concentration (which, for me, is usually working in a theater or behind a computer/console).
Tim, I think you are right that our work in time-based media is a factor—perhaps not only because it is time-based but because our work during recording/editing/mixing requires full concentration and detailed listening, our brain clock is more active and accurate.
Lord knows the neuroscience-savvy members of SSD will have something to say about this :).