Here’s a shot I took, erm, a year ago, playing around with ultra long exposures. Or, to be more precisely, simulating an ultra-long exposure by stacking many individual exposures.
On the shooting side, I simply set my camera to manual mode, 30s exposures in continuous drive mode and used a lockable remote release to fire off one shot after another (having disabled the dark-frame subtraction function as missing out 30s between each exposures produces ugly dotted lines rather than smooth trails).
Then, to “merge” the images, there are a fair few programs available. A well working, free and cross-platform one would be StarStaX.
As you can see in the little EXIF table below, this slightly confusing and anything but pretty photo was taken over the course of 11 h, out the back from our bedroom balcony. A bit of math tells you that’s more than 1300 exposures, all in the spirit of experimentation…