That depends on a lot of different factors. Let me tell you about three mushrooms that I added at the exact same time, about 3 months ago.
One was blueish, one green, and one red. All were about 1/2" to 3/4". They all shrank. Seriously. Thought they were just gone, they got so small. Then one day, boom! The green and the red were an inch wide, and the blue was back to 1/2", but now purple.
Over the next week or so, the green and red got to be about 2-3", and the purple is almost 1".
Like most soft corals, and anemones, they can inflate and deflate quite a bit. So actual growth is difficult to measure. But, when happy, they grow fairly quickly. 1/2" to almost 3" in just a couple months ain't bad.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef