3 weeks, 2 weeks, 1 week the day before?

I thought it could be nice to know what works and what doesn't.

Your question is incomplete. How long before start date of what did who apply for what?

Under the regs for most visas it's a maximum of ten days, TN-1 is a bit fuzzy because it's not a visa, it's an entry category.

To have a safety net, a canadian who needs to give two weeks' notice to current Canadian employer only after getting approval for TN visa, a 16 to 18 days appears to be a good compromise.

If applied earlier, the person may be losing on useful employment weeks in Canada out of total 52 week available for TN visa.