A long time I would assume, most of what I've read is fairly fast growth to 6" then 0.5cm per year. In a link from another thread that viktor posted years back the european had never grown one past 18-20cm and assumed most over that in aquarium are wild caught at that size.
There's also speculation about several possibilities of sub species.
There's a black bufonis that gets to 12" that's pretty cool.
I'm probably gonna struggle growing mine in the 35g but we will see, I'm possibly gonna add a powerful powerhead as it's reported they are pretty reliant on high oxygen content.