Mysql To Mongodb Conversion Time?

I know this is a next to impossible question to answer; but is it extremely difficult to convert a Yii website that uses MySQL to use MongoDB ?

I’m not a developer, but the developer i hired said he could do this but can’t give me any estimation of how long it would take.

Would it be as long as it takes to build a Yii website (eg. complete rewrite) or is it a matter of rewriting several core functions?