How long do wild seeds take to grow?

Wild seeds, whatever what they will give you in the end, will always take seven days to grow up, as you said. It is confirmed in the Stardew Valley Wiki Crops' page :

Regardless of what ends up growing, wild seeds all take the same amount of time to grow to maturity (7 days, not counting the day planted).

This information is directly from the Stardew Valley Wiki

Wild Seeds can be crafted out of foraged plants once you reach the appropriate foraging level (Spring at level 1, Summer at level 4, Fall at level 6, Winter at level 7). Each recipe will produce 10 seed packets. When planted, wild seeds will randomly grow into one of that season's foraged plants. Regardless of what ends up growing, wild seeds all take the same amount of time to grow to maturity (7 days, not counting the day planted).