Put the amount of concentrate you wish to use in a beaker and add water
until you get the concentration you want.

I mix up a tray full and use it that day, or until it slows down too much.

I never keep it overnight because it tends to stain plastic trays.

Not very. It says nothing about being poisonous on the label, but I sure
would not drink it. I think 50 to 200 micrograms/day are essential to proper
nutrition, but larger amounts can be toxic.

It depends on what you want to do. Kodak recommend 1+3 (IIRC), but that is
way too strong for me. I do not normally want a color change, or if I do
(depends on the paper), a very slight one. So, depending on the paper, I mix
up 1+9 or 1+19.

I do not know if you need gloves at all. I do not believe Ansel Adams used
gloves, and he did not die of selenium poisoning AFAIK. I use some heavy
black butyl (?) gloves, but they are a bit clumsy. I do not use gloves for
anything else.

The working solution keeps, as does the stock -
I don't know if it ever goes 'off', it hasn't
in my lifetime.

Until evaporation becomes a problem. I found putting
the chemicals away to be easy since I started using
wide-mouth Nalgene bottles and a good funnel.

It doesn't produce 'toxic fumes' though it lets off some ammonia.

Once evaporated it leaves a red powder which, I suppose,
could become airborne and do something/nothing.  I always
find red splotches here and there the day after toning and
just wipe it up.

Selenium is nominally poisonous, as are all metals
A little bit of selenium is required for life and is an
ingredient in most vitamin/mineral supplements and also
shows up in dog food.  Don't drink the stuff; keep it
out of reach of the South Park crowd -- all the usual
common sense stuff.

SA [Standard Advice] is to use 1:9 for ~1-2 minutes.
Most like to pull the print before it starts to go purple:
put another tray alongside with water and an untoned
print [good use for discards].  Tone until you _just_ see
a color change in the light-greys when looking from one
print to the other.  W/O the reference print I find I
invariably tone too much: When I can easily see it
start to go purple it is too late because it has
already turned to a noticeable purple [sorta obvious
logic, no?].

You can tone for as long as you like.  Warmtone papers
can develop some really nice colors with selenium: I go
for this really rich/deep chocolate color.

I found time to be unreliable as an indication.  Toning
seems sensitive to paper age, paper type, time in the developer,
temperature and other stuff of which there is too much
of to try and keep track.

IRC Agfa RC isn't a good choice for toning, I for sure
don't like the results with Ilford RC.  I use Seagull
graded glossy DW.  Results were great with Agfa Portiga,
but that is long gone.

I don't use gloves with it, but then I don't have
my hands in it for long periods of time.  Tongs are
my instrument of choice if I don't want to get my
hands wet.

For gloves, I like the purple nitrile gloves.  I
have difficulty finding them in XL and need to
go to a medical supply store.

If you are just toning for protection and using a dilute solution plan on
one day in the tray.  If you are using a stronger mix I used to mix it with
HCA and keep using it until it just quit toning.  Last a long time, old labs
that brown toned Ektalure with KRST used to leave the tray in the sink and
just cover it between sessions.