[Maigrey]

IIRC, it was book five, when Harry was about to be thrown out of the Dursley house, and a letter came to the house, addressed to Petunia, saying, remember my last.

His last could have either been the letter he left with Harry when Harry was dropped off at their house 15 years prior, or when Dumbledore replied to Petunia years ago, when she was trying to get into Hogwarts, that she could not do magic, but be use in the muggle world.

Personally, I think it was the letter that Dumbledore left with Harry when he was dropped off 15 years prior.

[Corin Stormhands]

IIRC, it was book five, when Harry was about to be thrown out of the Dursley house, and a letter came to the house, addressed to Petunia, saying, remember my last.

Yeah. I think that's it.

I think I expected more to come of it. But maybe it's implied in the reminder that the protection vanishes when Harry turns 17.