I think we've discussed this matter a few times before, it certainly can take more than 1 day, in my case the flight was on a Wednesday, the miles posted on Thursday and the account switched to gold on Monday! So while you can claim that miles should normally post in 1 day, you can't say the same about the account getting upgraded (one would guess, when you say "never more than 1 day for me", that you are regularly switching between silver and gold..., but we're still slightly off-topic here ​​​​​​).