Usually, my task-queue far exceeds the number of hours I'm either willing or able to work. Across any given six-month period, I'm usually billing to three to six different contracts. At the beginning of each month, my company's contract coordinator sends me the maximum number of hours I'm allowed to work against each of the contracts I'm associated to that month. Most months, if I wanted to, I could probably work 50-75% more hours than would fit in your basic 9-5 work-week.
All of this is background to explain why it is I don't really have "free time" or a day that's so prescribed that I have a fixed or limited number of daily tasks that I can burn through and have free time. Basically, if I've used up what I can do against contract "A" on a given day, I can flip to burning hours against contracts "B" or "C". Side benefit, it also means that if I get on a roll and burn to my pay-period minimum number of hours with 1-3 days left to go in the period, I can just take those days off (great for mini-vacations).
The closest I come to "free" time are my "meeting days". Those are designated days where I need to either be in the office for face-to-face meetings. Most quarters, this works out to about a 10-12 hours per pay-period. Since the structure of those days is mostly out of my hands, I end up having between-meeting dead-spaces. Unfortunately, when I'm actually in the office and people discover that fact, those dead spaces tend to get sucked up ("oh, hai: the corporate presence indicator says you're in the office! Got a quick sec to help me with/answer a question/have an impromptu face-to-face/etc.?"). Even when that time doesn't get sucked up, the connectivity at the office — particularly during core business hours — gets saturated (not to be unduly snarky, but usually by the people using their "free time" to watch March Madness, SCOTUS-nominee hearings, etc.) meaning that casting about the web for either constructive things or just mind-fluff is its own exercise in frustration.
On telework days, we tend to have a couple pre-scheduled telepresence meetings (phone and/or video calls via Hangouts/Chime/Skype/etc.) — one of the joys of geographically dispersed teams and customers. So, for sanity/continuity sake, one usually tries to reach a logical break-point before a scheduled call. So, that creates small breaks. However, those small breaks generally aren't useful for much more than Slacking, meme-trolling or hitting up social media or Ars/Cnn/etc.
On the plus side of such a time-framework, there's a lot of flexibility. If I choose to overload hours across a day or set of days in a pay period, I can then opt to take the balance of a pay-period's days off. That's far more effective for recharging the mental batteries than unused time in the middle of a given workday. Plus, having a variety of tasks that I can move across to "keep busy" means that I don't often allow myself the opportunity to get bored due to lack of things to do.
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