AS a rule of thumb 24 hrs at room temp would be my starting point. Bubbles will probably form throughout the substrate and that would show that some sort of fermentation has taken place. Slightly sour/alcohol like flavors also mean success.
For the short duration (24hr) mold forming will be minimal
and not an issue.
Yes to a little space at the top of the container.
Sealable containers which breathe would be fine. However you may find that with short duration ferments sealing is not required but that will depend on your surrounding environment.