I waited for Katrin to answer before answering myself.
We do not have an insight in the costs of mly. My guideline : one picture every second.
Even when the pictures overlap and connect, my mapper may complain that he does not see or cannot read the needed information.
It not only depends on the surroundings and speed but also on the quality of your picture and the field of view. On a bike you must take the corners slowly when you have set a 1 second interval.
Walking, you might consider 2 seconds, but then you must take your turns very slowly. You could try the distance mode and use the extra shot button in the corners.
On a boat on a wide canal, I might use an interval up to 8 seconds.
How the distance mode works depends on your hardware. With me it works totally else when I use an external GPS receiver. Distance mode is not so good in the corners. When Galileo arrives we wil need a sideways-acceleration dependant distance. That's why we should be able to switch to the time mode in a glitch.
I use 0.5 seconds interval when talking pictures sideways. I do not want to ruin mly either. I will redo the highways in the forward direction using the 0.5 seconds interval with a good camera and the sun from behind. This is not possible with my camera at this moment.
Reading the last interesting blog entry, I was also going to suggest 5 meters.