Low-medium

[frazoo]

how many hours of direct light is medium light?, low?, high?
will java moss grow beter in sand or gravel?
will it grow beter in only sunlight from 1 window shining at it 10 hours a day or a direct light 10 hours a day?

 

[George Farmer]

Lighting duration (photoperiod) isn't generally divided into groups like you suggest, this applies more to lighting intensity (see the pinned lighting thread).

Java moss grows best attached to something i.e. wood or stone. Although it can be grown on the substrate as a carpeting plant.

Sunlight from one window? Is it direct sunlight? This will probably results in a massive algae bloom.

Direct light - do you mean aquarium lighting? This is preferable to sunlight, it is more controllable and the light produced is less likely to cause algae.

A 10 hour photoperiod is fine. If you're only growing low-light plants i.e. Java moss then 6 - 8 hours is fine and will result in less algae.