Full immersion...how long til fluency?

[RubyOnRails]

Hi, just wondering how long it would take for 3.5 yo to become fluent....my son was born in Spain and lived til he was two. We sent him to a Spanish nursery from 14 months partly because we were certain we'd be staying in Spain....we came back last year and have focused his English, the Spanish kind of fell by the wayside. My husband is fluent and reads him the odd story, but I'm not so I stick to English.

If we return and put him into a Spanish nursery, how long would it take for him to reach fluency? He's bright but quite shy and I can't bear the idea of him struggling socially. He's had one year education in Spanish, one year in English.

As he has a head start I would think 6 months

He is so young, it will take no time at all. I moved to America from a non English speaking country when I was 10 And only knew a bit of English. Within a year, I was completely fluent with a strong American accent. I would think just a few months should be all it would take for your son.

I moved to a new language aged 5. Took about 6 -8 months for fluency. But it didn't matter too much before that, the kids were generally accepting and we still played together without many problems arising.