Beware of keeping it in a glass stoppered bottle.  Seem to remember that it
fuses glass to glass.

Depends on your competence, and whether there are untrained people at risk -
see the published safety sheets for guidance.

This is one of my old r.p.d. posts:

Often in labs, it is used as a 50 wt% (w/w, not w/v) solution
since it doesn't absorb carbon dioxide or more water to an
appreciable extent. The solution is weighed when a definite
amount of sodium hydroxide is required. In the instance
of a 50 wt % solution, one would use 2 grams of solution
for each gram of NaOH.

If you intend to use it in that manner, care must be used
when dissolving the sodium hydroxide. While dissolving,
the solution temperature can easily exceed that of boiling
water, and common containers made of plastic will melt,
and glass will crack. The solution is very caustic, and
can cause immediate damage to skin (deep ulcers) or blindness.

For "casual" use such as toners, the crystals are probably
more convenient, and easier to handle.

The 50 wt% NaOH solutions are available from numerous chemical
suppliers, university labs, and local supply houses.

If you decide to make a concentrated NaOH solution, consider
using a pyrex flask or beaker in an ice bath and sink to mix
the solution, and wear eye/face protection, heavy rubber
gloves and an apron.

To make up 200 grams of a 50 wt % solution, cool 100 ml
(100 g) of water in the flask, and add small portions (few grams)
of sodium hydroxide (total 100 g) over a long time period
(hours) to permit the solution to cool. The cooled solution
should be kept in a plastic bottle used for chemical storage
or perhaps one which contained liquid developer concentrate.
Label it, store it in a tray in case the bottle leaks,
and keep it away from curious people.