Is there a rule that prevents exploring every turn, do you need a certain % of land built to continue in a bulk explore?

You are not allowed to explore with more than 50% of your lands un-built.

if you try to explore with 80 turns but at some point in the bulk explore you don't have at least 50% of them built, you will see this:

"Exploration was halted. Your people demand you build on your existing land before exploring further. You are not allowed to explore with more than 50% of your lands barren."