There isn't a magic number for chargeability that will make you survive (or not)...
You need to mix in a vertical, level, pipeline, geography... For example in your first few years, you're still going to be learning the ropes & your chargeability is going to be hugely dependent on what work you're doing. No one expects a 1st/2nd year analyst to have the same chargability as a senior consultant or manager whose probably trying to grind out 85%+. Likewise, no one is going to give a sh– about a MD with a sh– chargeability rate because he's a rainmaker walking in $XXm/year in business. They are still having a big contribution, its just a different kind.
There is no magic number, higher = better but your focus should be on maximising your skills & being as useful as possible, especially if its early days.
IF/when (high churn rate..) you get sick of consulting, you'll also be glad you built up a decent (marketable) skillset, not just a high chargeability % when you want to make an exit.
now, regarding being on the bench and being laid off - I've heard of people being sent notices after 4 weeks on the bench (happened in 2009) & people being benched for 3+ months (happened to me in 2016).
I think it really depends on what their skillset is + what's in the pipeline. If the pipeline is bad or a sh–storm is coming, get off the bench onto anything. If the pipeline looks solid & you can see something coming up with a team you like / that suits your skillset & interests, you can probably relax a bit longer.