Timeframe to move to USA on GC approval for Canadian Citizen

Hi ,
Im a Canadian Citizen and was working in H1B visa .I got my GC approved recently .I use rent a house in US but i vacated recently as i work in border .Now living in Canada and commuting to work .
In the border the US customs and immigrations officers say i can take 1 year to move into USA.Though in the border customs and immigration officers told me , I dont see this time frame of 1 year mentioned anywhereofficially in any site or USCIS website.
Please advise

Thanks,
NF