I think the array expanding goes at about 4MB/s on my 3ware 9650 and I believe it was about 12MB/sec on Adaptec 5405... so you do the math.
I don't thing we are talking hours here.
Remember it is HEAVY disk trashing during this

It made me invest a whole lot more, and go for 8 drives at once. Didn't want to risk it going wrong in the process.