Guidelines for hardening off different kinds of plants

I’ve noticed that different kinds of plants are more resistant to sun scald than others. I’ve been hardening stuff off in a cold frame using week-long cycles of increasing sunlight exposure by 1-2 hours per day. Seems to work well enough. I’d like to increase the throughput and move things out a little quicker.

Are there situations where things don’t need to be hardened off? For example, if I move seedlings outside a few days after germination, do they need to be hardened? What sorts of plants need more hardening than others?

My Observations

Full week: Tomatoes, tomatillos
Less than a week: citrus trees, peppers

Citrus trees in spring need at least a week but in fall only a few days because the sun is so low.

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I usually do about 5 days and then put in the shade.