It depends on the bolt grade and the base material that was tapped, sometimes you need 1.5x the diameter or more, and sometimes less than the diameter is sufficient (a nut is not as deep as the diameter for example). I actually have a calculator in my app that will calculate the min thread engagement so the threads are as strong as the bolt or better.
Don't count the tapered end as actual thread. Put a nut on there and see where it sits down too.
Are those 3/8"? 3/8" grade 8 bolts into 50 ksi cast iron would want .56", Grade 5 would be .45"
Just keep in mind, anything less than that, and it's likely you will strip the block out before you strip the bolt if it fails. That doesn't necessarily mean it will fail, it just calculates what will likely fail first.
View attachment 321270
View attachment 321271