Is there really a big demand for this? I don't think Atari has a lot of nostalgia left to support this.
As for timeline, it doesn't take too long to fabricate a case for a stock circuit board, load it up with ROMs, Linux and a 2600 emulator. I could knock out a "working prototype" of this product myself in a day. A Raspberry Pi could do it easily and they're cheap as chips.
My cyncism for defunct brands revived by marketers apparently knows no bounds.