What Is the Mathematical Relationship between Time to Expiration and Theta?
The relationship between time to expiration (T) and Theta is non-linear and inverse. Theta decay accelerates as T approaches zero, following a concave curve.
The mathematical models show that Theta is roughly proportional to $1/sqrt{T}$. This means that an option with 1 day to expiration loses value much faster than an option with 30 days to expiration, even though the time difference is small.