I have a Novatron dynohub, a Planet Bike LED dyno light and a Spanninga (?) taillight. I find a little flicker for about the first 100 yards and then it's Ok. The Planet bike people told me this was a feature of their light. Actually it stays in flash mode for that time, then steady on. Taillight is almost instant on.

The stand light on both of these lasts a while. I'd say about 5 minutes or more. Although obviously not as bright as when moving.

This tends to be a big problem at work because there's always someone grabbing me to warn me that I never turned my light off.