Most female guppies you get from a pet store are already pregnant. But if the male is chasing her, she is going to be pregnant either way. Guppies give birth every 22-27 days, and are ready to become pregnant merely hours after giving birth.
From the looks of it she's probably either young and this could be late in one of her first broods, or she's a bit older and she's in the 1-2 week range. So I guess that she'll give birth in 1-2 weeks.
I wonder why tough corals are called hammers, and weak corals are called buttons...