You want a couple of seconds. Here are some pics I took to give you some ideas.
Less than a second.
About 1 second
Maybe 5ish seconds
These three were in the 8+ second range.
You just need to play around with it. It depends on how fast you are moving your saber. the slower you move the more light will be picked up and the brighter it will be. Also true with how many times you go over the same spot. This one was prolly 5 seconds of quick, stationary spinning.
Anyway hope this is helpful. I only have blue sabers so that's whats going on now but I'll get more colors soon. Good luck!!