Unless, of course, we define things more carefully.
How much of my time is actually productive output ? Well, a typical pre-pandemic workday is eight hours. Lose an hour for lunch. Lose at least another hour for tea and checking the news, emails, etc. Lose, say, about another hour on average for meetings and whatnot. That means I'm doing directly productive work for maybe five hours per day.
The theoretical maximum I can work in a day is 24 hours. So typically I'm working 21% of my full capacity : if I really do give 100%, I'll be working five times harder. Of course, my actual productivity will drop very rapidly indeed, probably close to zero after about 16 hours or so of this hellish torment, and within two days or so I'll collapse from exhaustion. If, somehow, I stay conscious, within a week I'll be clinically and permanently insane. Therefore, anyone saying they'll give it 100% - let alone 110% ! - is basically saying that want to die a horrible death, and should be carefully avoided.
On the other hand, we could define the maximum available working time to be eight hours. In that case, I'm generally working at 62% of maximum, so increasing to 100% is not such a big deal and quite sustainable for extended periods - days, certainly, probably weeks or even months. In fact even giving 110% becomes perfectly possible, as that only amounts to working 48 minutes extra per day*. The theoretical maximum output, by this definition, would be 300% if one works 24 hours per day, although this then becomes just as dangerous as before.
* Which somehow doesn't sound as impressive as saying "I'll give it 110% !".
Of course, after a while spending 100% or 110% of one's time on a single project is going to have adverse effects. See, that non-productive time is only non-productive if measured directly. In terms of educational value, for learning about other projects or other ways to do the current project, it's enormously valuable. If and only if one already knows the best way to get things done, and has no other important tasks to do, does saying, "I'll give it 100%" make any sense. Most of the time it's just silly.
Actually, my five hours of work is usually divided between at least two or three different projects. Right now it's three, so my direct productive workday fraction per project is probably more like 20% on average. I could then give it 100% for a particular project for a while, provided I don't mind telling everyone else to sod off while I achieve a pointless superlative benchmark.
Where it gets really tricky is when I'm thinking about a project but not actually doing anything. In terms of literally generating productive output - papers and code and the like - my output is probably a factor of a few below the estimated values, because much of this relies on unsuccessful tests and long periods of thinking about stuff. Without this seemingly non-productive time, nothing would get done. So is it really fair to call it unproductive ? Probably not : without eating I'd eventually die and not be able to work at all, so yes, absolutely, lunchtime counts as work. Expect me to be somewhere for eight hours but refuse me the option to eat ? Screw you ! This means I'm already working at 100%, plus or minus 10-20% since sometimes I leave early and sometimes I stay later.
And efficiency is even more complex. I can say, "I'll work extra hard on this project to really try every option", and this has some meaning, but I cannot possibly say just how more efficient I'll become. I might get lucky and hit on a solution in the first hour, but if there are lots of equally valid options to test, chances are it's still going to take a while.
Okay, so "giving it 100 [or 110] %" either means :
- Working oneself to death in about a fortnight
- Working an extra 48 minutes per day
- Skipping all lunches and meetings of any kind, even useful ones
- Making no changes at all.
Probably not. The announcement of "I'm giving 110% !" is, presumably, about working really hard to maximise productive output, not just working really hard for its own sake. But in that case, working longer might be a good idea, but it might equally well be a bad one. Working eight hours solid, without even a toilet break, is unlikely to end well. So 100% of maximum possible output could mean either working for a longer period each day but with more breaks, or it could even mean "I'm going to work a bit less". And if the goal is sustainability over a long period, that's probably a good thing. Sure, you can usefully work more intensely for a short while, but pretty quickly your productivity will go down, not up.
There, we can now finally drop that stupid phrase and say something more helpful instead. Tell your boss you're giving it 70% (or 25% depending on how you measure available time) and you can rightfully expect him to be damn grateful about it.