Good questions, Binastro, I'll be happy to answer.
I'm probably not a typical birder compared with most in other countries, but here there are many who do much the same type of viewing.
1). How long do you observe continually through your binocular at any one time?
- Anything from just a few seconds to maybe 10-15 minutes without a break. When I'm looking for raptors, I'll be in a bird tower or in a high place with wide vistas, have my scope ready at my side, pre-focused to about 3 km, and will be scanning the skies with the binoculars pretty much non-stop except when I or someone else spots something that needs to be identified with the scope. When scanning mudflats for shorebirds, much the same thing except binocular time is reduced and scope time increases. If the distance is 400 meters or greater, then I do much of the scanning with the scope.
Nearly all of my birding or astro binocular viewing is done with a finnstick.
2). What is the total observation time in any one day?
Depends on how much time I have available, but on days that I do go birding, a minimum of half-an-hour typically. Longest days can be close to a full day. During Spring migration, I may go to a good location around 6 am and leave around noon.
3). Does a change of focus help during the observation?
I'm pretty particular about focus as well as diopter setting. I follow what my eyes tell me, and if they are not happy, I'll try to tweak until they relax again.
4). How long is it before your eyes get tired?
My eyes don't really get tired from viewing with binoculars or a scope. In the past, I had problems going from prolonged scope viewing back to binoculars, but for several years now I've had an eye-patch type shade fastened onto my scope about an inch in front of my idle eye. That allows me to keep the idle eye open, whereby going back to the binoculars from the scope requires zero time to accommodate.
5). Are there any physical effects at the end of the day?
I have had problems with elbow pains from too many long days of holding binoculars during Spring migration. Pre-emptive use of anti-inflammatory muscle rub has taken care of that lately. It also helps to keep fit.