In a simple pendulum, if the iron sphere is replaced by a wooden sphere of the same mass, then the time period:

We know, the density of iron >  the density of the wood
∴  the volume of iron < volume of the wood since, density = mass/volume.
∴ It concludes that the length of an iron pendulum is less than the length of a wooden pendulum.
We know that, $T \alpha \sqrt{l}$ (Time period is directly proportional to length). 

Hence if the length increases, the time period increases. 

So, if the iron sphere is replaced by a wooden sphere of the same mass, then the time period increases.