When the pile of stuff pushed to the back to make way for new stuff becomes too unstable.
The big secret to keeping things reasonably tidy is for everything to have a place - this means that tidying up is quick & easy, as it is not hindered by frequent 'now where shall I put this' conundrums.
Accept that some thing will always live on the bench, and allow space for this- the whole back of my bench is lined with these:
for all small hand tools, so they're always within reach and quick to put back. These naturally organise themselves into most frequently used closest over time.
If it's quick and easy to tidy you'll do it more often - even things like getting into the habit of putting away one or two things while waiting for an iron to heat up can make a big difference.
Having the 'places' organised such that the stuff used most frequently is closest to hand also helps - the further you have to move stuff to put away the less likely you are to do it.