Good question. There's a ton of parasitic drain these days. I DON'T think I'd go a full month without a tender. Under normal conditions, the F30's alternator only charges during decelleration or coast, not 100% of the time as we're used to. Perhaps this changes if the battery is low, but I don't know for certain.
I personally wouldn't go past two weeks without a trickle charger. At least until there's more feedback about how well they manage their charge.