1) When the seed falls off when the branches are shaken.
2) Bamboo seed usually loses viability rather quickly, so the seeds should be planted quickly.
3) Storing the seed in your refrigerator can prolong the amount of time it remains viable. But I would recommend distributing and planting quickly.

Instead of shaking the branches, you can also gently touch the seeds and if they fall off they're ripe. It's a little more controlled than shaking. You're not trying to pull the seeds off, just knock them off if they're ready.

I had a few nitida seeds that I kept in the fridge over the Winter, and got about 30% germination. I'm not certain how many of the seeds were viable to being with, as some of them were pretty small/thin. The nice fat seeds are the good ones.