Time to see if I can get a good feel for the costs of the various kinds of goldmage magic. I really should have done this a long time ago, but I remember trying and getting results I couldn't make sense of, then never working through that problem. So, trying again. Ok... Apparently, a fully-scheduled goldmage who never needs to do more than pause can live past 50 (Chapter 7 prologue). If they use almost no magic before ten and we assume normal lifespans of ~80 - probably optimistic given the tech level, but they do have whitemages - then that means 40 years of basically perfect work burns away 30 years of lifespan. We were also told for the cost ratio of coexistence to freezing (10x-20x).
We have another data point, though. Apparently a ten-year-old has the potential for ~36 hours of coexistence (Chapter 3, Meea's prodigal sister). For the purposes of this calculation, I'm assigning pessimistic values to the costs; we will assume that the one who had *not* jumped was killed, and that there had been no other magic used. That works out to about two years per hour, or two months for five minutes, or about a day every five seconds (all values in Earth units). Freezing is a tenth as much (for maximally pessimistic costs), actually going forward or back is somewhere in between (and probably not the same). Apparently kids can *maybe* manage as far as a week into the past, so call it a decade per day, which is five months per hour, which is five days for two minutes, which is an hour per second (roughly 1/5th the cost of coexistence, or twice the cost of freezing). All of this is of course very wishy-washy (though the numbers turn out quite nicely with only a little rounding, unless I screwed up my math).
Huh. That's actually a less-atrocious rate than I expected. Meea *probably* lost no more than a decade or two in that misadventure, and possibly much less (the time from when that servant gave the utterly unhelpful message to come with him to when they were in the room and the jumped self was killed is probably less than an hour?) Pretty sure she lost years, could have been decades, and she's spent some magic before (and some since; if she froze for 25 minutes when she was fleeing then she lost another month doing that). It's not unreasonable that she could live to 40 or so, though, if she avoids other magic *extremely* well now. Given the sort of adventure she finds herself on at this time, though, that seems highly unlikely.
Of course, then we come to the thing that was probably the reason I had trouble with the numbers in the first place: apparently jumping backward and going on *really* severely caps your lifespan. As in, "jumping backward is very expensive. Jumping selves who've gone on don't usually make it past thirty even if they do scarcely any pausing, even if the jump was only a minute" which introduces a huge discrepancy into the concept of how jump costs work, since that should have only cost a few minutes. Plausible explanations time!
1) Lifespan is non-linear. The rate at which you burn it decreases drastically as you age, so if you take two hypothetical goldmages who have never used their magic, and one is 70 (ten years left, if nature takes its course) while the other is ten (70 years left), the 70-year-old would have way less than one-seventh as much lifespan as the ten-year-old. There are variations on this, of course; does a six-year-old who has burned the vast majority of her lifespan through coexistence use up the rest at the typical six-year-old rate, or at the rate as if they were actually 76 now? In any case, if this is the actual explanation, the derived costs above can be disregarded and the computations will need to be made more complex. This could explain the bit where it's said (of costs) that "we don't even know for sure if they're the same for everyone".
2) Jump costs are non-linear. This seems like the most plausible explanation for the thing where jumping back two minutes and then going on caps your life expectancy at less than half but it's even *possible* to jump a few days back. For example, if the simple act of jumping back - regardless of duration - imposes a massive cost and then you pay the duration at a more sane rate. 20 years up front would work into the values that we have fairly well; they'd need to be damn careful about who they ever send back, but most goldmages who are under 30 could manage it if it wasn't far. This does a lot to explain why Meea was removed from the position of bodyguard; she can no longer be certain she has enough lifespan to jump back by a millisecond, even though she could go on pausing and such for probably years of normal duties.
3) Various other explanations, such as that "lifespan" is something that you use up as you live and die instantly if you run out of, but it's not correlated with actual life expectancy. That is, non-goldmages who live longer than normal and then die of heart attacks at 90 died because of physical causes, not because they'd used up their hypothetically-could-have-been-200-years lifespan. In that case it's really hard to tell whether there even is a typical amount of lifespan, much less what it is, with the data we have.
Anyhow, I invite speculation (and corrections, either to my math or to my assumptions).