To what extent, if any, can I reuse mineral spirits?
As I touch on in a previous Answer, you can re-use rinsing solvents a few times.
The larger the volume you work with the more you can re-use it. And it is well worth rinsing in a much larger jar than you would perhaps naturally go with; small jars should definitely be avoided even if you plan to wash the brush in soap and water afterwards. Products containing resins — including varnish, "Danish oil", "Tung Oil Finish", "teak oil" and numerous others — are resistant to getting clean from a soap-and-water wash after rinsing in spirits. So think mason jar rather than babyfood jar :-)
In addition to pigments settling out, the 'solids' in varnish will also settle out of solution to some extent given enough time (days to over a week). This can be seen as a yellowish residue at the bottom of the jar. Oils will tend to stay in solution, hence the need to only re-use spirits a certain number of times.
Contaminated spirits tend to be easy to spot as they're not water-clear any longer but instead have a slight straw or amber colour.
If you would prefer something more concrete than the colour you can test empirically for whether it's time to discard your solvent by putting a drop on a clean sheet of glass and leaving it to evaporate.
- Fresh from the container any solvent will leave nothing at all behind on the glass.
- Slightly dirty you'll get a faint greasy or sticky film.
- Very dirty and you'll see a noticeable ring on the glass and the residue can be quite tacky. At that point you really have what amounts to thin varnish and it should be dumped.
You can also used a glossy glazed ceramic tile for this test. Do not use polished marble or granite.
It is possible, given their similar nature, that the answer would apply to paint thinner as well.
Yes, exactly the same principle.
Some sources claim this applies less to lacquer thinners and other 'hot' solvent as they can tend to hold solved resins/binders in solution rather than let them settle out, but I don't know that this is actually true. Those using lacquers tend to be pros and extremely careful about cross-contamination and general cleanliness in their equipment, so they may be being overly conservative. And they're not paying directly for their own solvents as leisure woodworkers do.
FWIW from my own observations I've seen solids settle out of lacquer thinners in what appears to be exactly the same way as it does with spirits or turpentine.