Mama - I think I took issue at your word "fluent". It clearly does not take anywhere near that amount of hours per week to become fluent. Reading your situation it appears to me that you would like your child to be as bilingual as is reasonably possible, which will take a lot more time and immersion than the input required to become merely fluent.
Both my nearly 4 year old and nearly 9 year old would be described as bilingual (the Mandarin I mentioned above is the elder's third language). Neither of them would have averaged anything like 20 hours per week in their second (non-dominant) language.
In my experience, which includes over 25 years of teaching, working in, managing and owning schools teachings MFLs, and working with bilingual (implementing their second language programme) and international schools I would definitely say that the 20 hours a week you quote is more thank enough to become a functional bilingual, as long as this continues for a fair few years. In my opinion, getting the motivation and input right is much more important that the number of hours.
NB: after writing my earlier post I did check a couple of bilingual/multilingual sites and I didn't see a mention of hours, but they did both suggest more hours than I believe is necessary.
Also, I am very "fluent" in a foreign language, but my knowledge of that language is intermediate level at best.
Heather- thanks for the link. I shall consider buying this.