If I keep mixed up Stop in a Datasafe 1L container, how long will it keep for in the container before going bad?
It is hard to say, because the problem is with mold, not exhaustion.
And again how many uses do I get out of the mixed Stop?
Lots and lots - probably more than you will be able to take advantage of (due to the mold issue) unless you have another big batch of film to develop.
It is probably most affected by how much developer carry-over you have.
And next time I'll mix up 1L of stop. How many ml of solution do I mix in with water to make 1L?
If you are only using it for a couple of rolls at a time, and won't be re-using it within a few days, mix it up at half strength and discard it after using it for those very few sheets and rolls.
Full strength: 1 + 19 = 20;
To make 1 litre, divide 1000 ml/20 = 50 ml concentrate + water to 1000 ml (950 ml).
To make 600 ml, divide 600 ml/20 = 30 ml concentrate + water to 600 ml (570 ml).
Half strength: 1 + 39 = 40;
To make 1 litre, divide 1000 ml/40 = 25 ml concentrate + water to 1000 ml (975 ml).
To make 600 ml, divide 600 ml/40 = 15 ml concentrate + water to 600 ml (585 ml).
For stop bath and fixer (and lots of other things) it is enough to measure accurately the concentrate, put that into the graduate, and then fill the graduate with water to the target total. Be sure to fully rinse the contents of the concentrate measuring graduate into the larger graduate, before you fill to the total mark.