Andy,
It all depends on the ambient temp at the time, if it is very warm, less time for each image, if it is cold and frosty go as long a you want
5 seconds is fine but 3 is OK as well it all depends on how quick your camera can download the data.
I used to take a 300 second sub at 400ISO with a gap of 3 seconds for download than a 300second dark, and start the process over and over, using a Canon 5D.
That ended up being a 12 minute image
This can make it a long night,
just to stand around in the frost , but the end result was worth it,
I have some fantastic Deep Space Images.
I did over two night 37 images of Eta Carinea, and that is one of my best results
but it was a total of 7.4 hours just standing around over that time frame.
At 5 seconds you will be fine.
Grab your self a nice bottle of red and enjoy the night
unless you have to go to work early
Leon