When is it too late for more Republicans to enter GOP primaries?

I googled this and couldn’t find answers to each states filing deadlines.
I ask this because if there is a Trump resignation or complete meltdown, I would expect several big names like Kasich and maybe Romney and Haley to enter the race.

* note....FYI....I am aware that 4 states have already canceled their GOP primaries to protect Trump and his baby man feelings. I guess they could always reschedule them, if needed.