You want to break it down into smaller sections - I usually use around 150km sections.
When you stop your Garmin recording (and reset it) at 350km, it will stop the course it is following as well. You will then need it to recalculate everything - this can take a long time if you are 350km into a 600km ride, and presumably use up lots of battery life as it computes what it is doing.
Obviously try and tie these sections with your controls, so you reach a control and set up the next stage whilst you are resting etc.
I might ride a 200km as a whole route, but probably not even a 300. It also has benefits if you've gone off route and need to recalc things anyway.
